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You can categorize these expenses - fees for using another website - in a number of ways. My recommendation would be "Advertising" (web ads) (see screenshot, below).

The IRS won't have a problem with how you describe these costs, as long as you are consistent from year to year.

If you want to provide additional detail (e.g. "3rd party Web Referrals"), you can enter them in the "Other Miscellaneous Expenses" interview of Business Expenses > Other Common Business Expenses. (see other screenshot, below)