Is it permissible to work in online product marketing for a commission (a percentage or a fixed amount per sale or per visitor), with the stipulation that the commission be paid upon reaching a certain amount, and after a specified period following the sale to protect the seller from fraud?

There is no objection to the described transaction. The website has the right not to pay the commission except on the actual sale after the specified period has passed, and therefore the worker is not entitled to anything before that. There is also no harm in stipulating that the commissions reach a certain amount before being transferred to their owner.