I have Woocommerce, UPS shipping and USPS shipping plugin activated on site. Things work working perfectly before the latest update of plugins. I am using Woo 2.6.3 and its addons.
After the update PayPal is not able to receive the shipping cost for the order and hence it process only product total without shipping cost and on site we receive an error as:
Validation error: PayPal amounts do not match (gross 2.75). Order status changed from Pending Payment to On Hold.
When I checked the log for PayPal in the request woocommerce does not add shipping cost under "payment_gross" and "mc_gross"
I have posted ticket for same to Woocommerce support but they will take too long to revert and as it's a live site I can't hold that much.
WooCommerce 2.6+ sends shipping costs as a ‘real’ shipping line item via PayPals API. If your PayPal account has shipping rules setup within it, these will take over and remove the cost WooCommerce sends across. So You need to remove all the shipping rule set from your paypal account.
Check below link for more details: