DomComp.com is another tool to see current prices & promos.
Rarely will you find a promo for a .com for under $5.99 for more than a single domain without a coupon, and those promos are very rare and haven't been around in awhile.
Registrars still have to pay the 'wholesale' cost for any .COM domain you register, which is currently $7.85. Usually when paying less than this, the registrar is covering the difference anticipating that you renew the domain with them at full price the following year.
Just an idea of how these promos work. Best.