I had to pay 1600,- thb! It's up to the IO and the connected agengy. The Immigration doesn't provide Visas at all. They send you to the agency next door. At least at Cham Yeam check-point it's handeled like this.
shipping to Thailand is offered by some Shops from overseas, but you can't have an Ebay Account or Ebay shop in Thailand. In fact, there's no Ebay in Thailand. What are you smoking?
Try to avoid DMK for the return. There are some Pitbulls @ Immigration. With 5 month you should be fine. If there are more than 180 days of stay within 1 year you might run in trouble and being sent back to Vietnam. All up to the IO.