Two products in the cart, one with a final_price of 84.5455 and another with a final_price of 62.7273 (as viewed in the order object). Click the PayPal Express Checkout shortcut button, instead of PayPal listing these two products and their prices, I'm presented with "All the items in your shopping cart" with a quantity of 1.

The PayPal debug log shows "Line-item subtotals do not add up", which is why this customer message is issued by PayPal. The issue is one of rounding.

The total price (present in the order-object's total) is 147.2728, properly reflecting the sum of the two products ... assuming a precision of 4. That value, when rounded for the upload to PayPal, is 147.27.

Unfortunately, the paypalwpp processing in getLineItemDetails determines the order's total from the in-cart products by summing each product's price after the price has been converted for sending to PayPal ... using a precision of 2. The sum of 84.55 and 62.72 is 147.28, resulting in the mismatch.