YOUR agent (UPS, DHL, FEDEX, Royal Mail etc.) have to pay the duty and VAT up front. If you have an account then it would have been cleared before departure from USA.
No account means clearance is handled in UK. Paper not electronic. YOUR agent will choose to get the money off YOU up front. There is a daily charge by HMRC for goods not cleared based upon declared value. Should YOU choose not to pay then YOUR agent still has to pay the duty and VAT. Hence the charge for clearing, they don't know if they are going to be stuck with an increasing cost because someone has gone on holiday etc.
No pay means YOUR agent gets legal right to the goods and just sends them off to auction. All they want is coverage of THEIR outgoings.
Now, there has been a lot of talk on this forum about foreign retailers billing Delivered Duty Paid. Yes, it does work. They use UPS, DHL, FEDEX etc who calculate, collect and deliver the tax to HMRC as part of the delivery fee charged by the vendor.
USPS just dump into Royal Mail or Parcelforce and they then become YOUR agent.
Stan