Shipping DDP - non-marketplace

Does anyone know who with/how you can ship orders to the EU DDP (Delivered Duty Paid) if the item was bought outside of any of the big marketplaces? I know this is an Amazon forum, but despite shutting what I could down (I’m on holiday mode here), someone in Germany managed to place a small order (under €150) on my own website.

My website is built on an American platform that none of the companies offering IOSS integration will work with and I’m not sure yet how I’m going to handle that. May end up having to setup 27 different price lists for each EU country and use a 3rd party service to handle the VAT.

But in the meantime, with Click and Drop and the Post Offices offering no way to pre-pay the VAT, I cannot see a way to send a simple large letter DDP. I’ve searched and searched all morning and have resigned myself to simply sending this one DDU (my website does warn that EU customers are liable for import VAT and any fees in the meantime. So at least the customer was warned). Just wish RM and the PO would hurry up and implement something.

Is there a solution I’m simply not finding?