Can you pay alternative costs like overload and also apply cost reductions from other effects?

When you cast a spell, you may pay an alternative cost instead of its mana cost. Alternative costs like Overload replace the normal mana cost entirely (CR 118.9). However, they do not lock out other cost modifications — additional costs, cost increases, and cost reductions can all still be applied to the alternative cost you are paying (CR 118.9a).

Cost reductions (such as those from effects like Goblin Electromancer, which reduces instant and sorcery spells by {1}) apply after you have chosen which cost you are paying — the alternative cost or the mana cost. So if you choose to pay the Overload cost, the reduction applies to that Overload cost (CR 601.2f).

The key limitation is that you can only apply one alternative cost at a time (CR 118.9). You cannot, for example, pay both an Overload cost and a Flashback cost simultaneously. But cost reductions are not alternative costs — they are simply modifications to whatever cost you have chosen — so they layer on freely.

Concrete example: You cast Mizzix's Mastery with Overload, paying its Overload cost of {5}{U}{R}. You control Goblin Electromancer, which reduces instants and sorceries by {1}. The Overload cost becomes {4}{U}{R} after the reduction. Both the alternative cost and the cost reduction apply together.

This interaction is confirmed under CR 601.2b (choosing alternative costs), CR 601.2f (applying cost modifications), and CR 118.9/118.9a (alternative cost rules), which together establish that cost reductions are compatible with any chosen cost, including alternative ones.