The U.S. Postal Service is known for working through rain, sleet and snow, but not on Sundays or holidays, right?

That is changing, quickly, through a deal with Amazon. In many parts of the country, products ordered from Amazon now arrive at your home seven days a week, year-round. As GeekWire reported last week, the rollout is expanding nationally even faster than originally expected.

The change is catching some Postal Service customers off guard, who may find it shocking that an institution of its size can evolve the way it does business. On Twitter, one person said it’s “kind of like magic,” and another person called it a #pleasantsurprise.

Sunday deliver from the U.S. Postal Service is kind of like magic. — Jon Schleuss (@gaufre) July 13, 2014

As postal trucks start circling U.S. neighborhoods, and customers get knocks on their front door on Sundays, people have become more aware of the service, and naturally, questions have started to arise. So we put together this FAQ to answer some of the most common questions about Amazon’s Sunday delivery service via USPS:

How come I’m seeing mail carriers on Sunday and holidays? What’s up?

In November 2013, Amazon.com announced it was working with the U.S. Postal Service to deliver packages on Sunday and holidays, starting in Los Angeles and New York, and in May, it added 15 more cities. In recent weeks, it has rolled out to even more cities, including Seattle, Portland, Northern New Jersey; San Diego; Fort Worth, Dallas and Houston in Texas; Richmond, Va.; Washington, D.C. (plus Virginia suburbs).

As for holidays, Fourth of July was probably the first time in many markets where people saw postal carriers delivering packages by foot.

Amazon and the U.S. Postal Service plan to continue to roll out Sunday delivery to a large portion of the U.S. population this year. Previously, the U.S. Postal Service only delivered Priority Mail Express orders on Sunday.

Is this a trial, or something more permanent?

It will probably last for at least five years, according to a WSJ report, which tracked down portions of the contract between to the two entities, however, either party can terminate the deal with 30-day notice.

Is Sunday delivery only for those who are willing to pay $99 a year for Amazon Prime?

No, Sunday delivery is available to all of Amazon’s customers. However, a Prime account holder can order an item as late as Friday and can potentially expect to receive it on Sunday. For example, I placed an order on a Friday recently, and Amazon guaranteed that one of the four items would be delivered on Sunday. To my surprise, it was delivered on Saturday, or just one day after I placed the order.

What can non-Prime customers expect?

Non-Prime members also can receive shipments on Sunday. For instance, a friend ordered an item on a Monday, and picked standard shipping. Amazon promised it would arrive three to eight days later, and it was delivered on Sunday, six days after placing the order.

How much does Sunday delivery cost?

In both the scenarios explained above, Sunday delivery happened to be free, but generally, non-Prime customers pay standard shipping rates, and it’s free to Prime customers. For either customer looking for something to be shipped faster, expedited rates would apply.

Is it available for all items?

No. Amazon says vaguely that “millions” of items are eligible when available in your region. The most common items include baby supplies such as newborn apparel, books and toys.

How do I know if a package will be arriving on Sunday?

You many not necessarily know until after placing an order. That’s when Amazon provides a summary of items purchased and dates for guaranteed arrival. In my case, a infant toothbrush for $6.99 was guaranteed to arrive on July 13. Amazon did not provide special notice that it was a Sunday.

I ship my Amazon orders to my office, which is closed on Sundays. Is there anyway to opt out?

We have not been able to identify a way to opt out of Sunday delivery. We have reached out to an Amazon spokesperson for the answer, but have not heard back.

Others just feel plain bad that their postal carrier is having to work on Sundays. Should we feel guilty?

The Sunday work is performed primarily by “non-career employees,” which is lingo for part-time employees. A U.S. Postal spokesman said these employees most likely would not receive as many hours as they do if it were not for Sunday delivery. Because of this, you will often see them not in uniform.

Dozens of job openings are listed on the U.S. Postal Service’s website, calling for temporary help on Sunday. Wages are listed at roughly $15.30 an hour, so if you think creating new jobs for people is a good thing, you probably don’t have to feel guilty.

Is this the first unusual relationship that U.S. Postal Service has inked with a private business?

No. The USPS recently contracted with Staples to have some services sold in their stores. The deal received considerable backlash from workers, who claimed it was shifting jobs to low-paying, less experienced workers. The last we heard Staples was abandoning the trial.

If the USPS is considering canceling Saturday service, then why would it add Sunday?

This is a good question with a very long convoluted answer, based on calls I made to people in the postal business. The agency lost $5 billion in its most recent fiscal year ended Sept. 30, 2013. The loss was largely attributable to massive healthcare funding requirements. Meanwhile, operating revenue, excluding a $1.3 billion non-cash change, increased to $66 billion compared to $65.2 billion in the same period a year earlier, representing the agency’s first growth since 2008.

The postal service has acknowledged the arrangement with Amazon will represent only “a small percentage of total Parcel Select volume.”