Is your WIP set in the 18th c? Because bread would get moldy pretty fast, with the lack of preservatives and things. When I make homemade bread, it starts to show mold if it sits for three days-- and that's in a climate-controlled house. It's more likely to be used the day it's made, or perhaps made the evening before in time for use the next morning--- but not being stored from week-to-week.
Some friends of mine are Greek Orthodox-- they attend church at two different churches about 500 miles apart. In both cases, I believe the ladies of the congregation coordinate to make the altar bread used for communion, although I'm not familiar with the ins and outs of antidoron/prosphora.
I think Catholic hosts are made from flour and water only, and Orthodox altar bread can sometimes have salt and yeast in addition--- yeast, of course, being a leavening agent. But the point being, if you're set in the 18th c, it's more likely that the ladies of the congregation are bringing in the altar bread fresh as-needed, rather than
sending away mail-order like they do nowadays.
Consecrated altar-bread would presumably be kept reserved in a tabernacle, if you're talking about Catholics/ Orthodox/High Anglicans/Lutherans. (Catholics stopped reserving it in an aumbry after the Council of Trent, but the holy oils continue to be kept in aumbries.) Reformed churches generally don't do "reservation of the elements" in the 18th c.