BM+FM for newborn below 3 months

Dear mix feed mummies How do you tackle FM wastage? Each FM hav to be consumed within 3weeks and most of the brands sells 800g packaging.. Since we are mix feeding, the FM will always go to waste before its even half finished.. So sad.. As much as i want to fully BM, sometimes i feel like my BM is insufficient for my baby, thus i have to top up. Or when my nipples still feels sore after the last feed which was 2hra back haha :( #1stimemom