The mice can't get past the wards, only the owls. I think the plan is

1. buy mice
2. find owls
3. cast animal friendship, using mice as food
4. feed all the mice to the owls
5. shrink
6. ride owls into house
7. find book
8. deploy portable hole below book
9. push book into hole
10. fold up portable hole
11. escape
12. huge profits