This doesn't read well and I'm not sure if this is correct:
Shipping fees can reduce the number of items a customer purchases, and they can decrease the frequency a customer makes purchases from the site.
What is the correct way to use the word frequency in this context?
The reason it "feels" like it doesn't read well is it seems like there should be something linking "frequency" and "a customer". However I do not know the rules of grammar well enough to know if there is a rule about this for this type of usage or how one would define the rule.
Update: the accepted answer introduced the concept of an "adverbial phrase" which is what I "felt" was missing but didn't know how to articulate.