Unknown line item on combined invoice

[global_philately]

I'm preparing a combined invoice for a customer in New York who has purchased six stamps from me. Under the postage cost on the invoice is an amount of $5.36 with no annotation next to it.

Can anyone explain this? I thought it may be a Sales Tax charge, but I can't make it fit

[jellybirddesigns]

Does seem a bit weird as the online sales tax for NY is a touch over 8%, but $5.36 is more than 10% of the total. Unless there is something else in that tax structure other than sales tax.

[global_philately]

It's something I haven't seen, and being in Australia, and selling globally, I would have thought it would have come up before.

The lack of any text is confusing, so I may need to lodge this with support. Wish me luck with that!