Can you spend multiple goods for multiple discounts? I've searched around and can't find this anywhere. The rules say "You may spend one good to decrease cost by 8"
If you could spend multiple I would think it would say "spend goods for a discount of 8 each". If it was definitely restricted to only one I would expect it to explicitly say "spend one and ONLY one good for an 8 discount"
Only one good.
Also the game mentions tou can't reduce stuff below zero which seems an odd thing to mention if you could only spend one good and everything costs more than 8, unless there are ways to reduce stuff further that I haven't seen yet?
Yes, there are other ways to reduce costs of things. (For instance, advisors can have -X Build or -X Upgrade that reduce costs)
Also, if you can spend multiple, does the "not below zero" rule mean I could use 2 goods on something that notmally costs say 12 and I just get it for free without the rebate or does it mean I couldn't use 2 goods at all because that would dro0 it below zero?
If discounts would bring the cost of something below 0, you can still apply all those discounts and just spend 0.