Postage re large letters is proved simply by maintaining a properly constructed postal register.  The Supreme Court accepts such registers as proof of postage.  As such any court or tribunal of lower jurisdiction, including any quasi judicial authority such as an Ombudsman, must also accept it as proof of postage.

Therefore if an item is not received but postage has been properly recorded in the register, and if PayPal refuses to accept it as proof of postage, as all of PayPal’s decision are subject to judicial oversight by the FOS, all seller has to do lodge a dispute, after which PayPal’s will immediately reverse its decision.

Been there.  Done that.  Haven’t lost one yet.