Honestly, I would find another supplier. Having small, malnourished prey items seems to be the trend with yours. If you aren't able to get correctly sized, healthy, well-fed, chubby rats every week, then you might want to go back to mice. At least you can buy the mice 10-20 at a time, feed them up, and then feed them off.

Oh, and weaned rats should be over 50g, if they are not, then there is a BIG problem. They should also be 4-5 weeks of age. Mine at that age, when I have a litter around, are usually over 100g and absolutely huge. They shouldn't be smaller than an adult mouse, like what you're describing. Baby rats are FAT, like baby bears or baby seal pups. Fat fat fat. If they are skinny, then they aren't getting the proper nutrition or they have a health problem.