There are two main reasons why buying soda in dollar shops isn’t a good idea. The first one is, obviously, the quality of the ingredients. The second reason is that they aren’t that cheap at all.
If you pay attention to your local supermarket’s prices, you can easily find a brand-name soda that will cost you less than a no-name dollar shop soda. Keep in mind that sodas are bad for you and that you should avoid them altogether.