Verify your card is 5x7 inches. If it's any larger, an A7 will be too tight.
Ask the associate if they have "5-and-a-quarter by 7-and-a-quarter invitation envelopes." where to buy a7 envelopes in store
If you are mailing them, ensure the paper isn't so thin it’s see-through. Verify your card is 5x7 inches