u/Broxxx_Lubberkin

Hello, I tried sending a parcel through a drop of box. Its a pair of climbing shoes. From Bristol to Prague (Czech Republic). All went well sending and a couple days later i got a small piece of paper with no real information other than "the sender did not pay full postage", asking me to pay 3.50£ (i paid the full price online before sending). When I paid that online a couple days later my package got returned to me. I did measure the package and it should be under 500g, but theres a chance it might not be. Could that be the reason for the return?

What should I do to not repeat the same mistake (assuming the mistake is on my part)