It depends on the reason. Your customers are protected by the Consumer Act (R.A. 7394). If the product has hidden defects, you are not allowed to refuse.
You cannot force them to accept an exchange. However, the customer is free to accept the exchange. The customer has the right to choose whether to accept an exchange or accept a refund.
You cannot force the customer to accept a store credit, and you must be ready to return the payment in the same form it was made (i.e. credit card/debit card/gcash, etc.)
No. You are only required to return or exchange the item if there are hidden defects.
Generally, there is no fixed time. It is advisable for you to create your own store policy and inform the customer of a reasonable period for returns and exchange.
If the product was mishandled by the customer, or otherwise damaged due to the fault of the customer, you are not required to return or exchange it.
Yes. The Department of Trade and Industry informed the public many times that underwear cannot be returned or exchanged after it is purchased.