In the UK customs forms can be filled out online, there are plenty of post offices in shops, supermarkets, pubs even. The Royal Mail and other postal services such as DPD and UPS even collect your items form your house, so I can't see how the UK is stuffed in that respect.
As for importing goods from the EU, I regulary order sax repair materials from the EU - there will be no change financially or otherwise for me unless I order goods worth more than the value of £135.
Goods over the value of £135 should not have the VAT of the eu country applied and instead should have the UK Import duty applied this end - for some eu countries their vat is as high as 27% and some are as low as 17% . I mostly order from Italy where their VAT is 22% So in effect I should be saving 2% in VAT but paying the bonding fee from the courier of around £8 - 11.
If I order goods with a total value above £200 from Italy or any other eu country with a higher VAT rate of 20% I'd actually be saving money and the VAT goes to the UK and not the EU. And if IM ordering From France where the rate is 20% I should lie only paying the £8 to £11 bonding fee, unless the company I am buying from slaps on a "brexit fee" which then IMO they're cutting their noses off to spite their faces.