Thank you for your cooperation.
This is a very common closing to a business letter (and it’s also a cliché).
Here is the typical situation: You have asked your readers to do something you know they don’t want to do. They have neither the time nor interest, but you need their help. You give them the order, but to sweeten the deal you then add, Thank you for your cooperation.
Can you see how insincere that sounds? They are going to cooperate only because they have to (or else).
Generally, in business writing you want to get to the point and get out as quickly as possible. However, this situation calls for more words so your readers will understand you know you are imposing on them.
Try something like this:
Jackie, I know you are exceedingly busy this week, and I greatly appreciate your taking the time to help me out with this project. Thanks so much for lending a hand; I’ll do the same for you whenever you need my help.
Five words or a whole paragraph? If you write the paragraph, your co-workers will likely have more respect for you. That’s important.