This card has minor errata to make casting the spell optional. When the ability triggers, first you choose whether to pay . If you paid, then you choose whether to cast the spell.

Thunderblade Charge triggers only once per combat damage step no matter how many of your creatures deal combat damage to a player, how much damage is dealt, or who that player is. (It might be you if the combat damage is redirected.) However, the ability could trigger twice in the same turn if there are multiple combat damage steps due to first strike or double strike.