Don't be surprised.
Has a new order just come in? Wipe that shocked look off your face. Totally fine if it’s your first, second or even your third one – but once you start getting into double figures, it shouldn’t still be so surprising that people actually want your stuff.*
What's more, when orders and re-orders become a thing, you can then start to tell other potential buyers that your cards really do sell. This saves you from saying vague stuff like, “they have been going down well in this one shop I have them in” or that, “they are very popular with people at work”. That way you will probably keep getting orders. And you never know, there may be a time when you actually come to believe your cards are more than worthy of being stocked. Well let's face it, no one re-orders crap, do they?