Parcels shipped via a shipping company or a courier service are routinely routed to customs for examination, where as postal parcels with a low declared value apparently rarely get checked. THB 8,000, however, probably does not count as a low value.
There have been news, though, that Thailand wants to start collecting at least VAT for all shipments valued at THB 1.00 or more, including for postal parcels, if they have not already started. In your case, though, there is the added complication of the classification of the product as "medical/massage"
Unless the courier can convince customs to return the parcel to them for return to the sender, I see little chance for you to recover the money.