If the declared value of the package is below the Government's limit, then DHL, Fedex, UPS, TNT and good old Royal Mail just deliver them - no cost. If the valuation (or as I suspect) the lottery as to which conveyor belt your package gets put onto means they have to do work, on your behalf, then you get charged because they could not just deliver it - it had to be recorded, documented, duty checked, VAT paid and that costs them money, so you pay. If your Fedex parcels arrived free, then that was simply luck, or a very low value declaration.
DHL for example, now have three separate systems.If the parcel feeds into some of their hubs and they know it requires VAT paying, they might even phone you up from Hong Kong or China and get you to pay. They got obviously loads of phone slam downs when a foreign voice says they are DHL - so now they give you in advance their waybill number and you can log onto DHL's UK web site or use their app to pay the duty. Normally though, you get a message - through the app in my case, and again, you make payment. Other times, you get a card through the door asking you to pay. UPS will accept payments for this at the door, but again prefer you to go online. Royal mail do the same. DHL will usually require on-line, while TNT might deliver the goods, then send you a bill. They don't accept on-line, or telephone, and want you to pay via BACS, which is tricky when now as my bank checks a/c and sort code with the name, you discover the account name (which they do NOT tell you) is Fedex - and you put the waybill number as reference. That works really well.
Getting VAT receipts for all of these payments is a total pain. DHL show you the VAT on screen and the £11 handling, but if you pay it, the receipt screen has the total only, so if you need the VAT showing, you MUST print the screen before paying. Daft really.
The other thing to consider is that many of the Chinese firms put totally random value on the customs declarations and even strange descriptions. An £800 order could be shown as £200 on the customs declaration. Of course you must notify HMRC so you can correct the underpayment, and we all know exactly how to do this and absolutely do it every time this happens. It's very inconvenient.