Overload and snapcaster costs

[KobeBryan]

are you still permitted to cast the overload cost with a snapcaster?

[iamajellydonut]

No.

Overload is an alternative cost. Snapcaster Mage enables flashback based on the card's mana cost only. Much like you're not able to flashback Force of Will for a blue card and one life, you are not able to flashback a card for its overload cost. There's even a Gatherer ruling on this.

10/1/2012: You can't pay other alternative costs of a card you're casting from your graveyard using flashback. For example, if the instant or sorcery has an overload cost, you can't pay that cost.

[cdr]

As iamajellydonut says, you can only pay one cost for a card. Snapcaster is giving the card a cost (a flashback cost, specifically) so if you're using that cost to play the card from your graveyard, you can't pay alternate costs like Overload. If you need to know whether a particular mechanic is an alternate cost or an additional cost (which you can pay), alternate costs say "you may pay [cost] instead" and additional costs say "you may pay [cost] in addition".