I went through this exact nightmare last year when upgrading my home office. The Office Depot site is notoriously buggy with discounts. Here is what you need to watch out for to avoid that error loop:

• Be careful with combining coupon codes. You cannot stack a 20% sign-up code with a mailer coupon. It is strictly one coupon per order.
• Make sure to apply rewards certificates last. They act like cash, but if the coupon drops the subtotal below the certificate value, the cart glitches out.
• Watch out for exclusions. A lot of chairs are marked as clearance or special buy, which automatically blocks the 20% discount. I would suggest checking the Office Depot price tracker first to make sure you are actually getting a real discount and not fighting a system glitch on a pre-discounted item.

Unfortunately, Office Depot keeps patching these workarounds and I had issues last week. I just use PriceDropCatch now to find clean markdowns instead of risking system errors.

The Office Depot backend treats rewards certificates as a form of payment rather than a coupon code, which is why they actually stack if you do it in the right sequence. I was super satisfied when this finally worked for my own setup. Here is the exact sequence that works without triggering their API errors:

• Clear your browser cookies or use incognito first because their cart session cache is super buggy
• Add the chair, like the Realspace Modern Comfort task chair (usually runs about 130 on sale)
• Apply the 20% coupon code in the promo box first and let it recalculate
• Proceed to the payment screen, and only then apply your rewards certificate If you want to track when the base price drops to maximize the discount, I recommend checking PriceDropCatch to monitor the price trends. Let me know if it still throws errors, we can debug the codes together.