Compare store credit against cash back on returns.
Store credit and cash are not comparable at face value. Credit typically carries a bonus but is only worth what you eventually spend, so it must be discounted by the probability you use it before it expires — retailers offer the bonus precisely because a meaningful share of credit goes unredeemed. Cash is worth its face amount less any restocking fee. Discounting one and deducting from the other puts them on the same footing, which frequently reverses the choice the bonus was designed to encourage.
Credit value
Effective credit = refund x (1 + bonus) x chance you spend it
Cash value
Cash value = refund x (1 - restocking fee)
Industry estimates put unredeemed gift and credit balances in the mid single digits to low teens as a percentage of value issued. If you shop there regularly, set the likelihood high; if it was a one-off purchase, be honest and set it low.
They generally cannot be applied where goods are faulty or misdescribed, and consumer protection rules in many jurisdictions restrict them further. Set the fee to zero if your return is for a defect.