If you know that you have the tendency to go over, then carrying extra cash should be a no-brainer.
Even with extra cash available, it's not always possibly for a provider to go over the time that you have both preplanned to spend together. She may have another obligation that you will make her severely late to, or even miss out on entirely. That could detrimentally affect her reputation with other hobbiests, and cost her unknown amounts of lost business as a result of a bad review or NCNS report.
If you know that you like 2 having 2 pops, and the second coming routinely goes into overtime, it's better to schedule it that way from the get-go. You will be avoiding the pressure you face to (re)produce within a short amount of time, and avoid offending the provider by shortchanging her or messing up her schedule.
In situations where she allows you to go over and you're not prepared to compensate her on the spot, you can always send her a comparable amount via an Amazon gift card. It's secure to pay using your CC & no info other than her email address is necessary.
Most girls love to shop, so I'm sure anyone would appreciate the gesture.