Ready made formula advice

Hi, this is probably such a stupid question but here goes.

At the moment I have to supplement with formula because my supply is not keeping up with my son, I'm working on getting my supply back up but like I said right now I have to supplement with formula. Usually in the day I just use powder but for the 5 am feed and when we go out, I use ready made formula.
My son is currently on 7-8 oz, mainly and preferably 8 oz but he will drink a 7 oz bottle and still be satisfied just maybe for a little bit shorter length of time. (I tend to give him 8 normally but this is my issue). The ready made formulas are only 7 oz and you can't buy bigger boxes. Like I said, he will be satisfied but I would prefer to give him 8 oz. I have thought about opening another box of formula and putting another oz in from that but that means wasting 6 oz of formula that has to be chucked after an hour and there is no way my son could drink 14 oz (2 boxes) in one go. I have also thought about making an ounce of powder formula and adding it too the ready made but again, I'm not sure if I can do that?

Is there a way I can up it an ounce without opening another box of formula and wasting it? Can I add an ounce of powder and boiling water to ready made milk?? Sorry if this is the silliest question, I'm at a loss on what to do. Can't wait for my milk to buck it's ideas up haha!