How do I keep orders accurate when it gets busy and complicated?

You have gotten orders accurate under normal conditions. What still trips you is the crush of high volume and all the special modifications piling up at once. That is the genuinely hard part. One order is easy. It is the flood, with a dozen changes each, that overwhelms the mind's ability to hold it all. The mistakes there are not carelessness. They are what happens when you try to hold too much in your head at speed. The fix is not trying harder to remember. It is leaning on a system so your memory does not have to carry the load, a consistent way you read, mark, and confirm each order the same way every time. Pick one point to build a check into, maybe repeating each modification back as you enter it, and do it exactly the same on every order, slow times and slammed times alike. A habit holds up under pressure where memory cracks. Make the routine automatic when it is calm, and it will carry you when it is chaos.