I just did a brand new fresh install of v1.5.1, with demo data for simplicity of having a product to test checkout with, turned on PayPal Express Checkout by entering credentials, then created a gift certificate and sent it to myself. Going through checkout allows me to use that gift certificate to reduce my balance to zero, which then automatically bypassed collecting any payment ... it just took me directly to the checkout-success page. No attempt to hit PayPal at all, because the balance was zero.

Maybe there's a problem with your Reward Points module?