Just go an answer from Tebex support:
Hi there,
Thank you for getting in touch with us.
I just want to make sure that you also take into account the gateway fee which is dependant on the gateway used, you can find these on your payment methods page:
I will outline all the fees associated with this transaction:
Platform fee for FiveM Store: 15% of $36.30 = $5.44
Gateway fee for Paypal: 2.49% of $36.30 + ($0.40) = $1.30
The $6.30 is taken off for VAT and then the $5.44 + $1.30 is deducted from $30 leaving £23.25
I hope this has helped clarify what fees there are and how they are applied.
If you have any other questions or if there is anything else I can help you with please let me know.
Kind regards, Tebex Support