When should I send thank-you notes?
Jot and mail a card as soon as possible after receiving a gift. Penning hundreds of personal notes in one sitting could drive you batty, so at the very least, try to write them in stages. But also keep in mind: If you’re swamped with responsibilities, etiquette dictates that you have a year to finish note-writing duties, just as guests have a year to give gifts.
Even though it might feel like cheating, sending a single note for a shower and a wedding gift is perfectly acceptable. Reference both in your message, though, to assure the sender that you have received and you appreciate each gift.