Here's how to find your receipts for online payments:
Go to Manage Profile, and look for the heading "Invoices, Payments and History."
Click on: "Membership" for dues payments; "Donations" for payments to the Endowment, Fellows or Sustainer's funds; and "Invoices" for everything else, including meeting registration payments and store orders.
Look out for the "Filter by Status" line, which defaults to "Open"; you have to change it to "Any Status" to see your receipts.
Note that your receipts will only be attached to your profile if you were signed in when you made the payment.