。。
Yeah. I'm pretty sure there's a discount coupon somewhere that I haven't used yet.
Literal
Yes. Certainly not-using discount-coupon [also-も] somewhere [at-に] exist [should-はず].
A brief affirmative monologue fragment. 確か ('certainly, for sure, if I recall correctly') is hedged confidence: 'I think I'm pretty sure but not 100%.' 使ってない is the contracted 使っていない ('not using / not used'), functioning as a relative clause modifying 優待券. も here doesn't mean 'also' but 'there are (things) too' — softening the claim to 'somewhere there must be.' ~はず ('it should be so') reinforces the speaker's confidence. 優待券 is specifically a 'preferential/discount coupon' for shareholders, members, or special customers.