Meiling doesn't have enough to pay the bill, perhaps?
Most likely, yeah. Either never looked inside the slip, or we'll get a flashback of her doing a mini-spending spree before getting back to the mansion.
Remember when Meiling couldn't pay for that super-big ice cream in a previous strip? Letty forced her to work as a result back then, and whoever owns this café is going to do the same.
Also, Meiling should've checked the contents of the envelope before "using" it...