Question:

Are handling fees charged to customers who return goods permissible in islam? Or is it akin to ribah? 

Answer: 

the enquired case, it is impermissible to charge handling fees and you are obliged to return the entire amount to the buyer. And, to charge a handling fees is similar to interest which is banned in Shariah, since the handling fee is not in exchange of anything; hence, it must be omitted.i

AND ALLAH TA’AALA ALONE IN HIS INFINITE KNOWLEDGE KNOWS BEST