From what I understand, in the past, BillMeLater was a One-Click purchasing option credit account that etailers could utilize much like the ill-fated MS Passport accounts.
A user would apply for a credit account exactly like a credit card with a credit line/limit and terms of agreement but there was no physical card involved. It would allow a user to sign up for one payment/billing option via BillMeLater and the user wouldn't have to go
through the whole ordeal with each vendor of choosing Login/Password, Enter Address info and Payment Info.
You just pick the item you want, click BillMeLater, the Vendor gets the users Name and Address to ship to, and their payment from the credit issuer just like a credit card.
It has absolutely nothing to do with "Oh Mr. Vendor plz send me item now I pay when feel like, kthxbai!".
Now the only reason I speak of it in past tense terms is because I do not have any idea of what/how it may have changed since being acquired by PayPal.