Facebook Marketplace and Facebook groups in Thailand are listings and chats, not an international shop. Many sellers have no public checkout URL. They expect a Thai conversation and a Thai address.
Mailbox
A mailbox works only when you can pay and the seller will ship to your suite address. Many will not ship to a warehouse unless someone confirms the order in Thai. If that is your case, use the Thai address the same way you would on Shopee Thailand.
Buy For Me
The usual path is Buy For Me. Send the listing link or a screenshot, the price, and how the seller will release the goods to our Thai address. We buy that listing. We do not recommend a group, a dealer, or a page.
If you want us to search Marketplace for an item you have not nominated, that is procurement, from THB 10,000 (approx $300) per order.
After it arrives
Intake photos, then the shipping calculator for the export price. Check what you cannot ship before you pay a seller for batteries, liquids, or replicas.