Ref. No. 1911/44-2263
In the name of Allah the most Gracious the most Merciful
The answer to your question is as follows:
If the shopkeeper already agreed upon giving you 2/% commission while he does not overcharge the customers, then this amount received from the shopkeeper is permissible as a broker's wages, and if it is not pre-agreed, then this amount is permissible too as a reward. Shopkeepers do this so that electricians can take their customers to those shops. However, if the shopkeeper increases the price of the goods, then it is not permissible to do so. And if the electrician doesn't go along, still he gets the commission then it's not allowable.
And Allah knows best
Darul Ifta
Darul Uloom Waqf Deoband