Removing a counter from a creature is an additional cost of casting the spell and may be combined with other additional costs or alternative costs that spell may have, such as casualty.

If you cast a spell with casualty this way, you can pay the costs in any order, which means that you may remove a counter from a creature then sacrifice it to pay the casualty cost. Bear in mind that removing a +1/+1 counter this way will reduce a creature's power, which might prevent you from paying the casualty cost with that creature.

Shield counters don't prevent players from sacrificing creatures.

Removing a shield counter in this way isn't the same as regenerating a creature.

If a permanent that would be dealt damage has more than one shield counter on it, that damage is prevented and only one shield counter is removed.

If a permanent with a shield counter is dealt unpreventable damage, that damage will be dealt and a shield counter will still be removed.

A creature with a shield counter on it may still be destroyed by state-based actions if it has damage marked on it equal to its toughness or has been dealt unpreventable damage by a source with deathtouch.

"Shield" is not an ability that creatures have and shield counters are not keyword counters. If a creature with a shield counter loses its abilities, the shield counter will still protect it as normal.