I buy 12 gauge trap reloads for $40.00 a flat and just keep them in the trunk of my car until I shoot them. I don't think that causes any problems.
In Africa, we always kept rifle ammo in the glove box of the land cruiser and that didn't hurt anything either. People say, though,that some PHs who keep ammo in their hunting cars for years or even decades end up having problems because the powder tends to clump together, reducing velocity. Also cordite, which has not been used for 50 years or so, was very temperature sensitive and pressures could rise if you fired it when hot. this is one reason the old British big bore loads were loaded to low pressures.
