Both the 'free' domain registration you receive with all hosting accounts and the bonus domain registrations you receive from promo-codes are 'free' for the life of your hosting. That is to say, as long as you keep your hosting plan active, you will not pay any extra to renew the registrations on those domains.
The 'free' domains basically work like this; Every year, just before your plan anniversary date, you are given one free 'domain credit' for each 'lifetime free domain' you have, you can use this credit to renew the domain free of cost, or you can let the original domain registration lapse and use your credit to register an entirely different domain.
These 'domain credits' do not show up anywhere in the panel (that I have seen), the only real way of telling that you have a 'domain credit' is that the renewal cost for a domain is shown as $0.00
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