Flashback and optional costs

[Malakai]

The specific question is:
If I target Shattering Spree with Snapcaster Mage's ability, can I pay the replicate cost on the card?

The more general question is:
How does flashback interact with optional costs?

[cdr]

There are Alternate Costs (a cost that replaces the mana cost of the spell), and Additional Costs (costs that are in addition to the cost of a spell) which may be mandatory or optional.

A spell can only have one cost, not very surprisingly. You can only pay the normal cost of the spell, or one single alternate cost. You can choose as many optional additional costs as you want, as long as those optional costs are applicable to the spell.

Flashback is an alternate cost. Replicate is an optional additional cost. You can look up a keyword in the comp rules (http://yawgatog.com/resources/magic-rules/) to see what it is, if you're not sure. You can replicate if you cast a spell using flashback.