Always write your letter or thank-you card to the customer on the SAME day that they come in; NOT the next day. Writing to the customer on the same day includes getting the letter or card into a USPS mailbox that picks up THAT night, NOT the next day.
The only exception occurs on Saturday. Letters to be mailed to customers on Saturdays must only be put into a USPS mailbox that is not scheduled to be picked up until the following Monday. If the letters are picked up on Saturday night, they may be delivered to customers on Monday and we are not here in case the customer wants to respond by telephone.
All thank-you cards are to be written THE SAME DAY AS THE SALE. Not the next day, or two days later, but THE SAME DAY. The only exception would be if the item was a gift and it would ruin a surprise to send a thank-you card to the customer’s house the next day. In that case, note that information in the contact log so that it is clear why no thank-you card was sent.