The secret to generating multi line CP Ro's is in the appointment setting. It is difficult to upsell in the lane for multiple reasons, the biggest being that the customer has a set expectation of how much time they have committed to their appointment, the upsell costs them money and time, and they will usually be low on one.
Using coupons to generate leads is great, dont be put off by not getting them on that visit, but ensure that the department has a method to upsell them on their next appointment. If you do your research you will find many solutions targetted at upsell on both inbound and outbound appointment setting.
I don't make a habit of publicly recommending solution providers, but if anyone would like to ask me about my personal experiences I would be happy to share my opinions on the providers i have used.