Cheapest priced copy (New Capenna Commander): $2.79 at Card Kingdom, near mint, their price list as of .
Contractual Safeguard 
Instant · Rare · New Capenna Commander · #115
Addendum — If you cast this spell during your main phase, put a shield counter on a creature you control. (If it would be dealt damage or destroyed, remove a shield counter from it instead.)
Choose a kind of counter on a creature you control. Put a counter of that kind on each other creature you control.
Legal inCommanderLegacyVintage
Not legal inBrawlStandardPioneerModernPauper
3 printings · $2.79 to $11.99 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| New Capenna Commander | 14 | Rare | $2.79 | — |
| New Capenna Commander | 115 | Rare | $3.49 | — |
| New Capenna Commander Promos | 14p | Rare | — | $11.99 |
7 rulings from Wizards of the Coast
- 29 April 2022If you cast this spell during your main phase, you may choose the shield counter for the second part of the spell.
- 29 April 2022A 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.
- 29 April 2022If 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.
- 29 April 2022“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.
- 29 April 2022If a permanent with a shield counter is dealt unpreventable damage, that damage will be dealt and a shield counter will still be removed.
- 29 April 2022Shield counters don't prevent players from sacrificing creatures.
- 29 April 2022Removing a shield counter in this way isn't the same as regenerating a creature.