So I assume that she offered to do this for you?
What I understand is that she probably feels useful or appreciated by "bying" things for you (this indicating low self esteem or even possibly narcissistic traits). However, even if you acknowledge this, you probably feel like you are using her in a way?
I guess your mum's gesture is bothering you for some reason, possibly as you said above as it may be indicating that "she knows best". I also think that what is bothering you is that you feel like you are using this "problem" of hers? So the question for you is really, do I acknowledge her efforts and make her realize that she does not need to prove herself in this way or do I make the best of this trait of hers?
Taking the risk of going a bit further and making a false assumption about you and your mum, I would also wonder whether there are any thoughts in you about love or the lack of it, such as " she is buying me this gift but she does not express love to me." " So If I accept this gift it means that I am not able to express my true needs to her and I feel angry for this".
The decision at the end is down to you. However, I would say that you need to be very aware of the reasons you accept it. If nothing of the above is valid and this is just a gift to you with no hidden messages or strings attached then there may be no harm of receiving it as long as you offer something equivalent to her at another time. You need to remember that money can have a very symbolic use in people.
Hope that helps