Some Orders amount 0£

I.m using Opencart 2.1.0.1 for my website and I'm having a big issue.
When a customer place an order, some of the orders (20% of them) have 0 as amount and no product in the order but the payment goes in my paypal account. Anyone have any ideea what it might be , I also contacted paypal and they checked and said is not definitely a paypal issue , but an opencart one.

Best regards, Alex C !