From a consumer adoption standpoint, the only thing standing in Amazon Register's way was a lack of marketing.

As a merchant, it was amazing. They offered promotional rate of 1.75%, which is basically impossible to beat for any small business.

For context, Square and most "simple pay per swipe" merchant service providers charge 2.75%. If you go out and get a really good deal from a intrchange+ provider, you might be able to get your rate down close to 2%. But Amazon Register got you below that with significantly less hassle, no subscription fees and a $0 equipment cost.

My wife used it at her small business for a year, and during that year I loved getting proposals from other banks and merchant service providers. Many had an offer like, "Let us audit your merchant fees, and if we can't beat them, we'll give you a $200 gift card."

I always gave them the opportunity to try, but with the caveat that there was no way they could beat our current rates, and I wasn't going to take their money when they failed. Mostly, I just never heard back after they realized I wasn't BS'ing about the fees we were paying.

If Amazon Register had been more widely promoted, they could/should have been able to get every credit-card-accepting small business in the country signed up at least for the promotional period. I'd love to see some kind of post-mortem to explain why they didn't.