I need to place somwhere class which produce Xml file and also class which is responsible to work with online payment services.
What is the best place to store them ?
Currently I placed them in Services folder, not sure if it's the best one.
any suggestions please
Honestly, it's up to you (or your team) which directory or namespace fits them best. There's really no convention that says one is better than the other.
Me personally I've seen developers use a Support directory for something like that, but Services or even Writers would work just as well!