Because if I don't — and especially, if I instead try to dispute the charge — I'll be severing my relationship with [big logistics provider], and so end up on a blacklist where nobody will never ever be able to send me another package through [big logistics provider] again.

Which would be especially annoying, because 99% of the time, when ordering things online, I don't get to decide — or even know in advance — what logistics provider the shipper will use to send me something.

Because it's a bill from the government (or a quasi-governmental agency), and they tend to be unamused if you don't pay those.