In an effort to increase its supply of certain iPhone models during the month of June, electronics buy-back service Gazelle is offering to pay an additional $20 on all working iPhone 4 and iPhone 4s trade-ins for the next 7 days.

$20 Bonus for 1 week only

To take advantage of the offer, which is exclusive to AppleInisder readers, simply use this link (or any of the pricing links in our Trade-In Price Guide) to active the $20 cash bonus before locking in a trade-in price quote online. The offer is only valid on working iPhone 4 and 4s models and expires on Friday, June 6th.

For those new to the trade-in process, Gazelle is a service that pays cash to customers who trade-in their old tablets, phones or computers by mail. It — along with similar services listed in our Price Guides — have seen a tremendous jump in popularity over the past two years as consumers have seized the opportunity quickly turn their outdated devices into cash to help offset the cost of upgrading to the latest Apple & Android hardware.

For example, a Hanover Research study completed in February 2014 found that 36 percent of Americans have now traded-in at least one smartphone online. That's up from a 23 percent penetration rate reported by a Gazelle study in June 2013, which was similarly up from just 6 percent in May 2012.

For its part, Gartner Research expects the U.S. electronics buy-back (or "reCommerce") market for high-end consumer electronics to reach $14 billion by 2017, as more and more consumers warm up to the concept.

How it works

In the case of the iPhone 4 and 4s, consumers who act now to lock in a trade-in offer on a functioning 32-gigabyte AT&T iPhone 4S will get a check for at least $120 cash, plus an additional $20 cash, for a total of $140 to help offset the purchase of a new phone (such as an iPhone 5s). A $32GB iPhone 4 in the same condition would pay out $90.

Customers have a 30-day grace period from when they first lock in a price for their old gadgets to when their devices need to be in Gazelle's hands. To begin the process, you select your existing device from the company's website and quickly assess its condition — broken, good, or flawless.

Within days, a pre-paid shipping box will show up on your doorstep. You place your old device in the box and send it off to Gazelle. (Alternatively, you can choose to use your own box and simply print a pre-paid shipping label.)

Once your device is received, Gazelle will issue a check within 10 business days for the value of your device. You can also choose to be paid more quickly via PayPal, get an extra 5 percent by selection

Current iPhone payouts