WHAT do subscriptions to a newspaper, magazine or Netflix account have in common? Once you sign up, you expect to pay the same rate every month.

Yet that’s not the case at Amazon when you subscribe to its Subscribe & Save program, which automatically refills orders for household staples like instant coffee, napkins or trash bags.

Amazon’s subscription program, which was introduced in 2007, lets consumers register to have their favorite consumables delivered regularly — monthly, for example — in exchange for a discount of at least 5 percent off each order. Buried in the e-commerce company’s terms and conditions is that the Subscribe & Save discount is applied to the price of the item at the time that the order is placed. And on Amazon, prices change frequently — including sometimes rising.

I learned this the hard way while reviewing an email summary of my Amazon subscriptions. A pack of lint rollers that I had subscribed to for more than two years recently jumped to $18.04 a pack, up from $12.44 since the last delivery a few months ago, or almost a 50 percent increase.