If that £68 is the 'declared value' on the document package, then I think that's the issue. I normally ship documents, including important documents, as having no commercial value (NCV). If that's what the lawyer entered on the shipping documents, then they eventually need to foot the bill for clearing the package.
Your alternative clearance costing includes, "Fee for amending customs documents: 1,321 baht" which suggests to me that the lawyer incorrectly declared the value.
Since UPS don't/won't provide customs clearance, and you don't want to decline the package, the only option is the alternative customs broker. Your lawyer should provide a detailed list of what the documents are, as they may still want to see that in their box-checking exercise.