You can purchase as many donation offers as you want, but they will be stored in the "Donation Packs" section until you send them to a specific player. For each donation pack, in the top right corner, you will see the number of purchased offers from that pack.

If you have purchased 2 identical offers, you will see a yellow button with "2" in the top right corner. If you haven't purchased from this offer yet or have already donated the purchased packages, the available number will be marked with a red button and will say "0."