We calculate shipping on the USPS website. There's a button on the left hand side that offers it. You go through, select envelope or package size, and then enter in the weight (we have a postal scale for this), and then it tell you how much each shipping option and how long it'll take.
We personally use First Class. It's the cheapest option, and it comes in 3-5 days. So it saves our customers some money.
We prefer paypal for payment on orders, it's quicker, and fairly secure. But we also do check or money order.