@Robin, Looks like the price that was actually paid was altered by the user (dynamically changing price is not allowed to protect the merchant from getting scammed). Some customers will try various techniques to pay a lower amount. So the plugin has a verification process to make sure that doesn’t happen. Who was this customer and and why did he pay an amount different than what you specified in the payment button? Send an email to this user and ask to get more clarification as to what he did. A customer paid but order did not go through. She received notice from paypal that she paid but there is no cart order record showing this transaction. [08/08/2020 3:32 PM] – SUCCESS :Checking price. Original price: 50. Paid price: 10 [08/08/2020 3:32 PM] – FAILURE :Error! Post payment price validation failed. The price amount may have been altered. This transaction will not be processed.
Having a shopping cart that supports multiple languages will make it easy for international shoppers to purchase from you, and takes a step in the right direction if you're trying to globalize your business.

Is there a way to specify the available number of items of a product that can be purchased within the shortcode?
Avoid carts with very low weight limits unless all you plan to transport is a set of towels and a few clothing items. A cart should be able to handle at least two heavy bags of groceries.