what would you charge to ship 4 one-size bamboo fitteds?

**Clarification** From what i understand about PayPal, you can send an invoice to the person buying the diapers and include the shipping price. If the invoice is sent before shipping (I have not shipped the diapers yet), how do I know what that price is? I don’t want to undercharge or overcharge, if possible. If someone who has done his before can walk me through this process, I would appreciate it!