Benjamin Franklin, in one of his less known pieces of writing, states that if you sell on credit you have given away one shilling in the pound.
That's 5%. It's pretty much the same today.
It's what makes credit cards SO useful, even if they're charging you 4%.
If I had my own shop, I'd be strictly on tngcas's plan; that and cash-basis accounting (as opposed to accrual).
I might never grow big, but I know exactly where I stand every day.