And H/t @pendletongs who tweeted about the Amazon announcement a few days ago
1⃣ (obviously!) Brexit and the introduction of customs formalities between the UK and the EU, and
2⃣ the removal of the low-value consignment relief which is an import VAT relief for low-value parcels.
Being by far the largest e-commerce platform impacted it could have taken customs formalities upon itself - technically at least cause well... we all know Amazon
UK here is a tip - just do what they do in Geneva. Find yourself a French friend across the border and get your Amazon goodies delivered to them