Are any of you using Plimus for annual support and maintenance fees?

If so, how do you handle the following scenarios:

1. Do you include support and maintenance as part of your initial purchase price or do you charge for this as a separate cost (optional or required) item?

2. How do you handle corporate customers that purchase multiple copies of your software throughout the year? Do you issue dozens of individual maintenance renewal invoices or do you pro-rate support and maintenance fees against a single anniversary date and then bill for all existing licenses on a yearly basis?

3. How do your determine the basis for your support and maintenance fees? As a percentage of a product's retail price or as a percentage of the product's actual purchase price (after quantity discounts and promotional codes)?

Thank you,
Malcolm