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this should cover it (emphasis mine)

601.2b If the spell is modal the player announces the mode choice (see rule 700.2). If the player wishes to splice any cards onto the spell (see rule 702.45), he or she reveals those cards in his or her hand. If the spell has alternative or additional costs that will be paid as it’s being cast such as buyback, kicker, or convoke costs (see rules 117.8 and 117.9), the player announces his or her intentions to pay any or all of those costs (see rule 601.2e). A player can’t apply two alternative methods of casting or two alternative costs to a single spell. If the spell has a variable cost that will be paid as it’s being cast (such as an {X} in its mana cost; see rule 107.3), the player announces the value of that variable. If a cost that will be paid as the spell is being cast includes hybrid mana symbols, the player announces the nonhybrid equivalent cost he or she intends to pay. If a cost that will be paid as the spell is being cast includes Phyrexian mana symbols, the player announces whether he or she intends to pay 2 life or the corresponding colored mana cost for each of those symbols. Previously made choices (such as choosing to cast a spell with flashback from a graveyard or choosing to cast a creature with morph face down) may restrict the player’s options when making these choices.

702.32a Flashback appears on some instants and sorceries. It represents two static abilities: one that functions while the card is in a player’s graveyard and another that functions while the card is on the stack. “Flashback [cost]” means “You may cast this card from your graveyard by paying [cost] rather than paying its mana cost” and “If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack.” Casting a spell using its flashback ability follows the rules for paying alternative costs in rules 601.2b and 601.2e–g.

702.94a Overload is a keyword that represents two static abilities: one that functions from any zone in which the spell with overload can be cast and another that functions while the card is on the stack. Overload [cost] means “You may choose to pay [cost] rather than pay this spell’s mana cost” and “If you chose to pay this spell’s overload cost, change its text by replacing all instances of the word ‘target’ with the word ‘each.’” Using the overload ability follows the rules for paying alternative costs in rules 601.2b and 601.2e–g