I try several times at using coupon codes on renewing domain names on namesilo.com. However, never succeeded. In the emails between email@example.com, they confirmed there is coupon code can be applied in renewing domain names.
My advice is just renew the domain in regular and save the time to earn more money: The retail price of renewing a .com domain name is $8.89. If you transfer to godaddy, the price is typically around $8. The time spend on transfer processes is worthy more.
Here are some options if anyone really willing to spend time:
JOIN DISCOUNT PROGRAM
If anyone has more than 100 domain names, then the price can be around $8.79/year for renewing each .com. The normal price is $8.89. More details please check on Namesilo Discount Program
TRANSFER TO OTHERS
The price for transfer in is lower than renewing. Sometimes godaddy will release some super coupon codes which give very low price. Go and check godaddy coupons if there is any.