If the seller is providing samples for their products, you will see an option to Order samples on the listing page under the Make an Offer button. Provide your shipping address and select shipping method. Then use a credit card to pay the shipping fees.
The money you pay is sent to the seller only once you receive the samples and complete the transaction in the marketplace.
When you receive the samples, you must go to the sample request you made ( in the inbox) and mark the transaction as completed. This step is mandatory for the seller to receive the shipping fees payment.