Airfare(s)

[Xinfu]

-Eligible expenses include: accommodation, economy airfare, transportation.
-Eligible expenses include: accommodation, economy airfares, transportation.

Is the plural form optional? If so, is it because the sentence is listing things?

[Rusty]

The plural form is optional. I would not use it in this list.

[Xinfu]

Thnak you. Is it because the sentence is listing things?

[Rusty]

Yes. Airfare is an eligible expense.

I just noticed that you have 'accommodation' in the singular form. It should be 'accommodations' when you mean lodging.

If 'meals' were in the list, it would also appear in the plural form, because you wouldn't want the eligible expenses to include only one meal.
A suitable alternate to 'meals' is 'food', always in the singular form.