Use of “frequency” in “decrease the frequency a customer makes purchases from the site”

Question

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.

Answer 1

Frequency is a noun, and you want to modify the verb phrase makes purchases, so you want an adverbial phrase or clause. We can provide this with at which or with which:

...they can decrease the frequency with which a customer makes purchases from the site.

We could also replace it the related adverb frequently

...they can decrease how frequently a customer makes purchases from the site.

There are other variations, but these are the two that make the least change to the original.

Answer 2

Although I do not have a specific explanation, I think that this sentence flows better:

Shipping fees can reduce the number of items a customer purchases, and they can decrease the frequency with which a customer makes purchases from the site.