Even just getting one bag at a time, these days it seems to be cheaper to do rodentpro than petco because they are so much cheaper per rodent. I think petco charged me $15 a piece for small rats while I was waiting for my rodentpro order, not sure in your area, where a bag of 20 plus shipping these days is only 3.50 per.

A small rat is way too big. For right now just get some more 18g mice to buy you some time as well as make sure she bulks up a tad before trying to switch to rats. I would consider buying a bag of rat pups from rodentpro or similar to arrive before she runs out of the next 3 pack of mice.

You could try finding a snake breeder in you area that is willing to sell you partial bags of rodents. That's what I did when I just had a couple snakes, found someone by posting an add in CL "looking for someone to go in on rodentpro order with me to reduce shipping costs". Might be someone here willing to do it too, could post in the classified section to see.