Allow creation "trial" products with auto expiry based on the amount of trial days/hours given for that product. No invoice needed, completely eliminate the invoice from the trial product.

BUT, allow the sales person, setup "Auto Offer" so by end of the trial period, client will be greeted with a paid offer to continue benefit the service/product.

WorkFlow:

1. Offer trial/free service for X amount of days/hours/weeks/months
2. End of trial period, auto trigger a paid offer (default package or custom one)