ReadonlyactionReadonlytierThe PriceTier to be added to the tiers field of the StandalonePrice.
The action returns an InvalidField error in the following cases:
* Trying to add a PriceTier with `minimumQuantity` < `2`.
* Trying to add a PriceTier with `minimumQuantity` that already exists for the StandalonePrice.
Adding a PriceTier to a StandalonePrice produces the Standalone Price Tier Added Message.