STOP! You are doing the right thing.... do some research first, as you are.
You will need at least 2 tubs...32L is ok for mice. You will need a mesh top (12mmx12mm minimesh from Bunnings @ $16) because if you melt holes in a tub lid, the mice will prolly chew thier way out of it. A weight on top, like a house brick to keep mesh in plase. You will need a small tip (mouse size) drinker bottle that can hang from the mesh (with a little tang & hole on drinker bottle) and breeders choice paper litter (woolies $18) or hi-absorb shavings ($15 pet or feed shop) for bedding. Dont use any clay or crystal type kitty litter in case mouse eats it, then snake eats mouse.... bad! You will need a little house in each (icecream container with hole in side). As the females become pregnant, put them in the tub without the boy. When both girls are pregnant, put 2 houses in girls tub. When the babies ween at about 21-24 days, you will need a 3rd tub set up the same. After the babies ween, give the mother a weeks rest, before putting back with boy. Mice usually sell around here for $6 each, twenty for 2 is too dear.
Dont do any deals like that with pet shop... they are ripping you off. It will cost you hardly anything to feed the mice, I feed mine woolies homebrand frozen (but thawed in microwave then cooled) sweet corn ($2.95 for 1kg) and peas ($2 for 1kg) This will last you about a month. You can also give the mice anything you eat.... rice...pasta....vegies.... carrots..... broccoli (cooked), bread crusts.... anything (but not meat) dog biscuits, cat crunchies (not too many too salty... but 1 or 2 each each day or 2)
You dont need to buy mouse/rat/guinnie pig mix.... they are expensive and your mice will do better on fresh (frozen) veg.
Tell the pet shop they can buy the mice for a dollar or 2... you will make a bit of money this way, if you do the work for the mices food, the petshop will make all the money.... not you!
Do you realise, for snake food, you will need to euthanise (kill) the mice babies? If so, you will need a soda stream system ($74 woolies) a plastic tube and another tub (@ 10L)
Best of luck, PM me if you need any advice
Will you be ok to kill them? If you live at home, will mum let you keep them in the freezer? you better check
Edit: Dont agree to supply the petshop 10 every week.... mice arent like clockwork! Assuming you buy 2 female (not pregnant) and 1 male, the mice have to be about 8-10 weeks old at least to breed. Dont let the petshop sell you some old mice as breeders, mice can only breed successfully up to a year and a half or so. Buy some youngish mice... preferably girls from one place.... boy from another (to avoid inbreeding.... 3 from one shop will prolly be brothers and sisters :shock
From when you get them home and put them together in 1 tub, you could expect your first litter in about 3-4 weeks. Feeding corn & peas (and other stuff) the eyes will open at about 2 weeks, they will ween at 3 weeks (+ a day or 2) When weened, you will need a boy tub and a girl tub (especially for ones to go to pet shop) Keep some female babies to grow and become breeders. SO, there is no way you can be contracted to supply the petshop 10 per week (or fortnight) you wont be able to supply that many regularly until you are well established (about 2-3 months all going well)
When you have live stock to sell, put an ad on IGA, Woolies & Coles notice boards & Gumtree. I would then advise you to subscribe (or become power seller) here on APS, to support this great site, and allow you to place advert on here.... then you will be all set
Again, best of luck
If you are living at home, get your folks to read this, I am happy to advise you (and them) as you go. Dont AGREE TO ANYTHING WITH PET SHOP!