I remember several years ago weighing the benefit of paying $8.50 per month for checkfree (via Quicken) or paying for postage stamps on all of my bill payments via US mail. It was a narrow distinction, but I decided I'd pay more promptly with checkfree.
Nowadays I send all my payments electronically for free or use a credit card on a website. I must make more than 30 electronic transactions per month, which would be about $12.00 in aggregate, plus the convenience of paying on my schedule vs. the USPS and not having to worry about correct postage.
Not a bad side benefit: convenience and cost savings rarely go together.