Buyee package returned without warning.

Hi. Might get long as I try to give as many details as possible. I'm really trying to figure out what actually happened here.

I recently had a package shipped from Buyee, a proxy company that buys things from Japanese markets and ships them globally. Along its journey I received a notice that 'Shipment is held pending clarification of goods valuation Pending Proof of Payment (PPY).' This was on February 12th.

The day of, I received an email from DHL with a form to submit proof of payment. I submitted an invoice with the values of the goods. I then received an email the next day (Feb 13th) saying this information was insufficient -- this was my bad! The page says invoices aren't accepted, and offers a list of other valid proofs of payment. I submitted these to the page: PDFs of email confirmation for each order and a bank statement, with relevant payments highlighted and labelled. I got an automated email acknowledging that these were received, as I did on the 12th with the invoice.

Basically, I haven't have heard anything back since then. On Feb 18th I called for an update; I was told basically "it's in customs, it's an opaque process and we don't really know what's going on, they [the UK gov] is checking it, hope it gets out soon."

On Feb 19th, Buyee (proxy company that sent the package) contacted me informing me they may request a return to mitigate storage fees if I did not reach out to them and keep them updated on the package. I promptly reached out and explained the situation.

Today, Feb 21st, without any email/call from DHL I see on the tracking page that the item is being returned to sender.

I called again. I was told "well, not my department, but it has to do with proof of payment [obviously!]... sometimes they just hold the package and return it, that's just a thing that happens..." and given a tracking number for the return package.

Fine, it's being returned, maybe that can't be stopped now. What I'd like to know is why so I can avoid this in the future. Were the details not accepted? Did DHL fail to provide these details to customs? Was it because Buyee made the request, despite me updating them? If so, I feel like DHL would be able to tell me that. I'm posting here in case anyone has insight as to the processes at work here.

Thanks in advance.