I assume you're leaving 'be' out of the example by mistake.
I would say:
- We're (supposed to be) getting lobster rolls from Mason's.
- We're supposed to get lobster rolls from Mason's.
However, it also depends on what you're trying to say. In the first example, it looks like you intend to get lobster rolls (or are currently in the process of getting them) in the near future but the parentheses hint that there is a possibility you might not get them (they might have run out, they've forgotten them in the past, etc.).
The second one is more definitive. It is either saying you intend to get the lobster rolls although you haven't yet so it is uncertain. It can also be seen as a definitive statement like getting the lobsters rolls is either expected of you or you're saying Mason's is supposed to provide them but there is some chance they may not.