OP, I just did a check with AT&T and found that you are correct that one can get AppleCare + at their store.....But that does not mean other places will offer this. Verizon last year said yes to me, but when they tried it, the Apple rep said no. Others have said no as well. If one is 50 miles plus from an Apple store, Apple will allow you to call it in for the coverage, but if under 50 miles, one had to bring it into Apple first to be checked over, which did not make sense to me, because the phone was purchased brand new and had not left the Verizon store.
Some have been lucky and Apple gave them the coverage, but they had to by-pass the normal Apple rep and go directly to AppleCare + direct line which is shown below. Unknown what will happen this year so be prepared to read from those that give advise all over the map.
800-275-2273 (AppleCare +)
I would also call Radio Shack. They should know, but not a guarantee. Some employees just think they know.