How should I manage my stock and sales?

1) I make the burgers -> I want to write that number down somewhere

2) I pack the burgers a few days after (in packs of 4) -> I want to know how many packs I have

3) I take the order from the client (i almost always sell 4 packs of since I have a promotion for that) -> I want to be able to have a product called "Promotion of the week" which has 4 packs. I also would like to customize this promotion for people who exchange flavours

4) I deliver all the promotions the same day -> I need to have the information on 3) on my cellphone

What are the best practices for this? If something cannot be done, what is a good workaround?

Thanks a lot

Gery