I have the same problem as you.
I ask her to go to wellcome. At the very begining, she has many excuses to show me that it is not OK to buy in Wellcome, eg.
--she tell me that wellcome don't have this and don't have that
--she has got all the recepit but just don't know why the balance was not correct
--she lost the recepit and the balance cannot be check correctly
After that I told her it was her duty to keep the money, if the balance was not correct or if no recepit was shown, she will have to pay for it.
For 買餸, she is OK now. But for other house work and cooking, she gose worse. And, sometimes she 買餸 come late and told me that the recepit was stucked in the cashier and she has to wait for half hour to get it back.