Not a scam but an expensive service to use for small items.

Can be a good service if shipping large items not available by a postal service.

Simply don't consider sellers using the GSP whether it be USA or UK.

I have messaged sellers many times if they would consider using USPS and were not aware they had been opted into the GSP.

Some sellers are happy to offer USPS or Royal Mail when asked.

But like I said, it's not a scam so don't set yourself up for defamation on a public forum.