The percentages you will get can be negotiated. When you are an authorize dealer, there are two ways they normally do it.
1. You get discount for wholesale purchase of their products EXAMPLES ARE WHAT YOU STATED ABOVE and i qoute " like when i buy like 200ghana cedis card then they will add like 20ghana cedis card"
2.Commission based where you sell a certain amount of products for a fixed amount of money. So if you sell say 200ghc of credits, you get x amount of money.
Now if i should give you a suggestion, your aim for this business should be a wholesale supply where you set up a shop at one location in the beginning and choose the discount option. The reason why i prefer the discount to the commission is that, the discount is not fixed. You can always negotiate your way with them and let them increase it anytime. Lets say first time you bought 200ghc mtn cards and you got 20cedis worth of mtn credits, the next time you can buy the same 200ghc and tell then to increase the discount to say 30ghc since the economy is bad or any other marketing tricks you can come up with. In this case, your ability to make profit will always be increasing compared to the commission that is fixed.
So when you have become successful in one place and have gotten some money down, then you should open another shop in another location and repeat the process.
Money is about value and scale. Lets say if you are making 2 cedis profit at your shop every day, if you had 10 different shops at different places, it means you would be making 200ghc perday. Now that should be your aim.
Either you have one shop that reaches a lot of people or several shops where one shop reaches a few.
So get your first shop up and running on the discount option and then prepare to expand later.