Can't say no

Often we have problem with saying no. This is one of the biggest problems for me. When someone ask me if I do something, almost always I say: Yes, it isn't problem, I do that. Problem is and it isn't a small, because we shouldn't make commitments when we for example haven't got time. Recently ( probably 6 days ago) when I was with my mum in the shopping center some woman walked over to us and asked " Have you got some time? Today we have reduced price for our cosmetics and maybe you want trial them in your skin?" We have a lot of time, so we agreed. This is our ( my and my mother) mistake. This " special offer" isn't very surprising and cosmetics were very expensive... In that moment our problem appeared. The woman advertised products very well and persuaded us to bought something. Of course my mother can't say no and bought that. We must say no in similar situations, but it isn't easy....
Oda 2a