AP renewal is $361.03 including tax. They should send you a renewal letter about a month prior to the expiration date but not everyone seems to get one. Receiving the letter is not necessary to renew the AP.
Not sure what Mouseport is but AP's are the same price with the same tax rate (currently 6.5%) included in them no matter where you buy them.
I would think the tax rate is the same since I beleive it is all inclusive in the final ticket price.
We did receive the letters, however I know you can renew by phone or fax. They will send you a voucher for redemption when you arrive. That is what we did.
Remember to bring the passes you are renewing with you. If you forget them, they have to look up things in the computer and it takes a bit longer (or so we were told by the last CM who renewed our passes).
You also have a 30 day grace period to renew in person. Since my AP expires May 31, and we arrive at WDW on June 2, I'll renew it then at guest services. Your expiration date is still back dated, of course.