You might be seeing the result of automating the customs forms. Up to a couple of years ago, the customs forms could be hand written. Now they are almost all created on line. Even if someone goes to the post office with a hand written form, the postal clerk has to create an electronic copy. This makes it easier for your customs offices to do their work. As you mentioned, it was VAT that they collected, but it could have been VAT plus customs fees.
The other big change is that USPS no longer has a low cost method to ship international mail. The smallest package pays the two pound rate. There is no package rate for one pound anymore. That means it costs about $22.50 to ship the smallest package first class package to the UK. Priority mail is more expensive and starts at $48.50.