PayPal customer service doesn't seem aware of the ticket we opened with MTS (Ticket ID 160801-000308), so we've been hearing that inquiries to them about this issue are being met with denials of any issue at their end and/or generic information about passing shipping amounts from a cart that's off-base for the actual problem. BTW, whenever you need to contact PayPal about any technical issue, we'd recommend calling 888-221-1161, which should get you to an elevated tier of support better acquainted with E-junkie and more knowledgeable than their regular front-line customer service agents.
We ARE passing precalculated shipping amounts from our cart to checkout, and as far as we've been able to tell, the problem seems to occur when their checkout site calls back to our system to recalculate shipping for the buyer's full, final shipping address. They seem to be sporadically either dropping the recalculated shipping amount we return to them in that callback, or dropping the precalculated shipping amount we'd passed from the cart as a fallback in case their recalculation callback fails or times out.
Affected merchants who have another payment gateway set up for card-based checkout may wish to disable PayPal checkouts temporarily until this issue is resolved. To do this, simply sign into Seller Admin and click Edit Profile, then un-tick the box next to Sell With PayPal, then click Submit to apply changes.