BigBuy uses money deposit (they call it Money Pot) to deduct order's amount every time you submit an order.
Your money pot residual amount is shown inside ApiDrop supplier's page https://merchant-staging.apidrop.com/settings/suppliers/bigbuy
When your money pot is 0 and you try submit and order, you will receive an error.
How to recharge your BigBuy money pot:
Inside your BigBuy admin panel https://www.bigbuy.eu/en/controlpanel/refund there is a section called Money Box where there is a tab named "Deposit Money".
There you can define the amount of money you want to deposit and when you press the button "Deposit" BigBuy will show bank transfer instruction.
Once they have cleared your deposit, a new and updated balance will be shown inside your ApiDrop account.
Great! You are now able to submit a real order to BigBuy.