[Symbiote]

Using the EU and UK systems is optional, but without using it the small business's customers might be annoyed by the handling fees applied by the recipient's mail carrier, and the delay it causes.

I have seen some foreign merchants (I think DigiKey?) offer the choice, as their business customers don't need to pay VAT directly in this way, and may well prefer to do the import paperwork themselves.

I haven't seen a choice for any large retailer (Amazon, eBay, Etsy, AliExpress etc). They don't want customers annoyed by fees, or returned packages from unpaid fees and duties.