Just got another report from a seller that sometimes the PayPal checkout site is not showing buyers any shipping charge at all nor adding such to their payment total, even when shipping was precalculated in the cart before checkout and the default shipping address from the buyer's PayPal profile was applied automatically.
This seems likely related to the issue discussed above, so I've added details about that to the ticket we opened with PayPal MTS about it.
All this, along with those screenshots @jsb sent us (showing stuff in PayPal checkout that we'd never seen before and could not replicate), seems to indicate PayPal might have rolled out a half-baked update to the way they show/apply shipping charges at checkout. MTS says they're working on it, so hopefully they'll roll back to the pre-update version or iron out the problems in the update forthwith.