Just not sure about the fee for a period of time
Me too.
Why? So you pay 0.05DRK when you want to mix, then you mix how ever much you want. If your client goes over 24 hours, it'll spend another 0.05DRK to keep going. It eliminates the timing attacks and double spending attacks.
People are used to registering for monthly or yearly subscriptions.
Maybe offer those too, and give the user some discount?
monthly 10% off
yearly 50% off.