If they use Paypal, they can use Bill Me Later. BML is integrated into PP. So, although you can't advertise that you accept BML, you can by default.
Basically, buyer with BML account chooses PP at checkout. Goes through checkout, when they get to the PP site, BML shows up as an option for payment. Buyer chooses BML, all you see as the seller is "Paid Via Paypal".
I spend a LOT of money on my BML account! LOL Most of my supply vendors take Paypal, and I just bill it to my BML account. Same thing when I was gift shopping on Etsy during Christmas.