For anyone in the UK looking for some cheap-but-decent solar panels that will do USB, Amazon has some 60W Eco-Worthy fold-out panels for a bit over £50 currently, which is about as decent as it gets at the moment. They're not full-on waterproof, but come with a controller for bigger batteries. A comment says you don't need a battery to charge USB devices through the controller, so in theory charging USB batteries should work OK too.
Caveat emptor though - I haven't tested them, so can't vouch for the accuracy of this. They look similar to my 60W set at least, which I'm really pleased with.
Discovered them after investigating PV supply chain ethics and transparency (my notes here) last week, when I noted they were on for anywhere between £70-100.