Hey Trapperjohn I do some wildlife but mostly work with another small company that I am happy to share work with. We usually charge once the work had been completed. We have an upfront cost for removal, an upfront cost for exclusion and also for trapping.
If I am just send the invoice, I don't have to make any service appointments but if I did I would make sure it is a reoccurring service, change the frequency to every day for one week then add or subtract days if the animal has be caught or removed. It's good to have reminders in your schedule just in case.
