Yeah, I made my expressed bottles up to a certain amount, I was expressing so usually ended up with slightly more milk than I'd need for a feed so just used the remaining for a feed. If you google "mixing breast milk" there are a few bits of information about how to do it safely.
I used these how you say, although I expressed so tipped the milk into the bottle I was using. I used one per feed and then it got sterilised. I also would mix my milk, so if I pumped more than I needed it'd go towards the next feed. I did this as long as the milk was from the same day, when I got up the next morning I wouldn't mix it with milk from the day before. I found them so helpful as I was wasting so much milk.
I am 37 weeks pg with #2 and thinking about feeding. I bf DS for 6 months but DH gave one bottle of formula every evening so I could get more sleep. I was happy with this. However, I also had a massive milk supply and leaked all over the place. In particular, when I was feeding off the first breast I'd leak enough from the other to get through 3 sopping breast pads.
I just read about breast shells and they seem great. My thoughts are I could use them to collect my leaking and use that milk in a bottle once a day. But I am confused about sterility. Can I collect milk throughout the day and tip it into the same bottle? This is not sterile, surely? Do I clean them / sterilise the shells after each time?