negative line items "how to" and subscription possibility

Hi,

I'm looking to allow a discount option via PayPal checkout to negate an initial setup (trial period) cost.

I have a product set up to have a trial period cost of $150 for a "initial" setup cost, and then a lower $50 monthly cost.

I'd like to offer to "waive" that initial setup cost to some users, is this possible with the new 1.5 support of negative line items, yes or no? How so?

Even if no...is there documentation I can view for how to use the negative line items in general?

Thank you!

-r