Yeah, some companies (wrongly) treat email addresses with plus signs in as invalid.

You could try to persuade the company to fix their systems, e.g. by complaining that you couldn’t enter your email address “amy+sam@” or something. But they probably won’t fix it. And then the best you can do is shop elsewhere and tell them why… or cave in and do things “their way”.

I don’t use the plus-sign technique any more. Instead: I use my domain and set up different email addresses for every organisation I deal with, but have them all configured to land in the same Inbox. But that’s harder to set up, unfortunately. You can do it with GMail, just about, though: set up e.g. yourrealname-shopping@gmail.com (hyphen, not plus sign) as a new email address and configure it to forward all mail to yourrealname@gmail.com (so you never need to log in to it). A bit clumsy, but it works.

Up to you if it’s worth it!