There are three options:
1.) Have a weed control contractor you work with exclusively who you trust who you can recommend to your customers. This contractor can give you commission for the referral or you can be satisfied with giving him the account knowing he will take good care of the customer. Try to avoid someone who offers what you offer as they may take the account.
2.) Work with a contractor who would act as your subcontractor. You bid the work or he bids the work. If you bid the work you should already know his pricing structure so you can be sure not to underbid the work. If he bids the work, have him submit the bid to you and then you have the option to add an additional fee to profit from the transaction or not.
3.) Shrug your shoulders and tell the customer to look someone up on the internet.
Suggestion #3 isn't where I would go.