I recently worked on a very similar problem at work, and although I am unfamiliar with R, I hope I can contribute. Given your goals I would suggest you use a Heckman specification. These models proceed in two stages - firstly, they estimate the probability of a sale through use of a probit model, and then derive the expectation of the sale, conditional on a sale being made. I am suggesting this because the value of the sale is very obviously dependent on customer self-selection. Sorry, I don't know how to implement this in R, but these kinds of models are widely known so there should be a package. 

Good luck.