If you charge the buyer for shipping or if you include shipping in the purchase price (free shipping) and you sell an item, the buyer should have already sent you the money to pay for the shipping label. You will use this money to pay for your shipping costs.
If you print an online label through eBay, the costs will come out of your managed payment account account, or you can change your eBay settings to pay for shipping and fees through an a separate credit card or PayPal account.
Alternatively, you can take your package to a shipping company, paying the fee onsite. We do our best to suggest a correct shipping amount. Sometimes, due to box sizes, packing materials and weight the shipping can be more or less.