Um, bad idea to keep any bread product under 40F until served. That's refrigeration temperature, and it just so happens that the process of bread going stale is accelerated at refrigeration temps. Better to keep the bread product at room temp, or freeze it. Freezing means the bread will go through the refrigeration temperature range quickly, so the staling won't be as accelerated. Same thing for thawing - just a small amount of time would be spent in the stale zone.
If you are worried about fillings spoiling at room temp, make the sandwiches the night before, freeze them (sealed in plastic), and then let them thaw (covered, of course) for about 1 hour or so before serving.
I routinely buy bread in bulk. That is, two big loaves for the price of one. I freeze one immediately, and then thaw it out when the first loaf is consumed. I can't tell the difference between the two loaves. Sometimes I'll freeze it in the original package overnight, and then vacuum seal it gently for longer storage. Not too much vacuum because it might squash even frozen bread, esp the lighter textured ones I favor.
PS-forget about adding STPP to a bread mix. Sugar, glycerine, salt, and oil help to keep baked products from going stale. But oils will make the bread more like cake, and salt will slow the rising process. There's always polysorbate and di-glycerides as humectants, if you can find them on the shelf.