We first reported the issue to PayPal MTS on Aug. 23. They finally acknowledged just yesterday that they've identified a relevant timeout issue at their end and are working to fix it.
This corresponds to what our Devs found when they enabled more detailed logging at our end, where we did not see any recalculation callback attempt logged at all for affected orders.
In that scenario, PayPal should apply the precalculated fallback amount(s) passed from our cart -- which appear to be getting reported correctly as tax
and mc_shipping
in IPN after payment -- but they haven't been adding that to the buyer's payment total.
So apparently their callback routine is sporadically just stalling out completely, failing to perform the callback and failing to add the precalculated fallback amounts, yet still reporting those amounts in IPN, causing the IPNs to fail the Suspected Fraud price-check we perform before processing the order.
If you'd like to offer buyers a different checkout method, this help page lists all the payment services we support5; any of those will add a separate checkout button in your E-junkie Cart for buyers to pay directly with a card.
If you do this and also want to remove the PayPal checkout option from your E-junkie Cart until they resolve this issue, you can do that in your E-junkie Dashboard under Manage Seller Account > Edit Profile > More Payment Options > Show PayPal in shopping cart (unfortunately that can't be disabled if you're using PayPal Payments Advanced or Pro).
Of those services, we'd recommend Stripe in particular if you're based in any of the countries they support2, as they're pretty quick to approve new accounts, very easy to set up with us, and don't even require buyers to enter their card billing address during checkout.