There are multiple issues:
For delays it’s a combination of Covid and Brexit border controls
The new method of handling import charges (over or under £135) is not Brexit related.
Import charges now becoming due from EU suppliers is Brexit related as we are no longer members, but pre Brexit we paid local VAT rates applicable to the EU country and post Brexit we pay UK VAT rates
For goods worth up to £135 the overseas seller is to register with HMRC, charge UK VAT at point of sale, declare all the details on the package and pay HMRC on a quarterly basis
(With EU purchases the tax implications on the price only changes a little to take into account any difference between their local VAT and UK VAT - in practice the price of goods and the price of postage may have changed)
For goods over £135 the overseas retailer just charges you their basic goods price & postage (either excluding VAT or they pocket the extra as they are exporting)
On arrival at the border it is evaluated for VAT and whether or not import duty applies
You get a bill for 20% VAT plus any import duty, plus a handling charge for the carrier administering and storing the package.
This is how imports from the wider world have always happened, but while we were in the EU the ‘border’ did not exist
www.gov.uk/government/publications/changes-to-vat-treatment-of-overseas-goods-sold-to-customers-from-1-january-2021/changes-to-vat-treatment-of-overseas-goods-sold-to-customers-from-1-january-2021