Appointments+ / Marketpress - How do you set a service with a variable price / addon price

I have client that runs a cleaning service, and they service 4 different locations (on separate days) and currently offer 4 different products at each of those locations..

This is all going smoothly so far, except the client needs to be able to have the cost of the service scale up with a variable in an additional field.

i.e product 4 is 'window cleaning' the base price is say $10, but they want customers to select how many windows they need cleaning in an additional field and the price to go up say $5 per window - hypothetically $10 for service, $15 if 1 window, $20 if 2 windows, $25 if 3 windows etc

Is there any possible way to achieve this with Appointments+ (with or without the Marketpress plugin and addon activated)?

Or alternatively, if it is not possible... I assume I would need to setup an individual service for each variable (service 1 = 1 window, service 2 = 2 windows etc) but if I do that is there any way to keep the services together in a window cleaning page that they could select which of those multiple services they want rather than have to scroll through dozens of window cleaning services?

Thanks in advance. Kade.