What is the ruling on working for a micro-service marketplace website that adds a 5% processing fee or a minimum of one dollar to the buyer upon checkout, to cover payment gateway fees and the website's operational and developmental expenses? And is it permissible to act upon something whose permissibility is highly probable without absolute certainty?

If a website requires a commission from its users in exchange for their benefiting from permissible services, then there is no harm in that, whether it is a known percentage or a fixed amount, and whether it is imposed on some users or all of them. The predominance of conjecture is sufficient for the permissibility of the transaction, and certainty is not a condition, for the predominance of conjecture is annexed to certainty, and rulings are built upon it.