Tiffin is early 19th century acording to my dictionary, and it can mean a light meal but although it might be of English origin, it's so bound up with the British Raj and India (and teatime) I think it might give a colour to the meal which you don't want.
I agree with Abernovo, the main meal of the day is dinner, and I think they'd call it that in c1810, not supper, which is a lesser meal. The courier is presumably of a lower class, so how about using a semi-dialect word like snap or bite for a meal on the run? The upper class lot would call their mid-day light meal luncheon, but the nouveau riche chap might call it lunch to try and use a new racy term that's just been coined to try and integrate himself (unsuccessfully).
I agree with Abernovo, the main meal of the day is dinner, and I think they'd call it that in c1810, not supper, which is a lesser meal. The courier is presumably of a lower class, so how about using a semi-dialect word like snap or bite for a meal on the run? The upper class lot would call their mid-day light meal luncheon, but the nouveau riche chap might call it lunch to try and use a new racy term that's just been coined to try and integrate himself (unsuccessfully).