The last few times my stomach has been upset when I have gone there but I'ved learned a few more tricks since then and I am happy to report that I felt fine this week when I went.
Have them make the sandwich on a plate and not the shared board. Also I just found out that the mayo has cross contamination. They use the same vat and knife for all mayo, so just ask them to use the packets of mayo in it. That should help you not get sick.