Breastmilk Storing

This is a really stupid question but the internet is giving me different answers.

I am feeding my baby 3 oz (50ml breastmilk and 40 formula). I have mammary hypoplasia and have a low milk supply. I pump and feed him what I pumped.

I have been pumping ahead so I have a bottle where I store what I pump for the day. I can only produce 2oz per pumping session. Someone told me I can't combine milk in one container if it's pumped at different times even if I feed him that the same day.

How should I be doing this? Don't want to waste the very little I have. Thank you!!!