Is it permissible for me to buy goods for a customer and add a percentage of profit to them? And what is the specific permissible percentage?

It is permissible to buy and sell goods to a customer under two conditions: First, the purchase must be real. Second, the goods must not be sold before they are acquired and moved from the store, due to the prohibition against selling goods before possession and acquisition. As for profit, there is no specific limit in Sharia. It is permissible to agree upon a certain amount or a known percentage, as long as the buyer knows the value of the goods. It is recommended to be gentle and not exploit people's needs.