You should provide an option to either permanently or by purchase opt out of getting any kind of "thank you" correspondence. The reason I'm suggesting this is because on two recent occasions, I or my wife purchased a gift and the thank you note ruined the surprise. The first time was when I got a new cell phone for my wife's birthday. The plan was to give her the new phone, and swap the SIM cards, but you sent her a text message before she got the gift. The second time, my wife bought a gift for me, and I received the email thanking me for the purchase before my birthday.
I understand that you want to show your appreciation to your customers, but I'm sure I'm not the only one to experience this unintended consequence. Please seriously consider changing your policy, otherwise I'll have to consider making gift purchases elsewhere.