If you signed in and paid for something on the AFS website -- whether it was a donation, an AFS membership or Section renewal, or last year's annual meeting registration -- you can find the receipt in your member Profile.
Sign in, then click on Manage Profile in the Profile menu. Scroll down to "Account History" at the bottom of the screen.
Receipts for AFS membership dues payments appear under Membership. Section dues payments are not here, but under Order History.
Donations, of course, will appear under Donations.
All other payments will appear under Order History, including annual meeting registration fees and AFS Section dues payments.
Event Registrations contains only the record of your registration form, not your registration payment, which is recorded under Order History.
You should always sign in before making a payment online at AFSnet.org -- that will speed up the payment process, and tie your receipt to your profile. If you aren't signed in, the payment will go through, but you won't be able to locate the receipt at a later date.