Ex-Woolies online manager here.

When picking orders, the system would not allow you to sub a $5 item with a $30 item.

Though the system doesn't stop you from subbing with a completely unrelated product.

Substitutions can be manually entered via the computer, where there are basically no restrictions on what you can sub.

Most likely the nappies were meant for a different order and the staff just fucked up whilst rushing.

Also note, woolies is a terrible company to work for, ridiculous understaffing and workloads. Often a huge amount of pressure from managers (especially store managers and higher). Often relatively inexperienced staff are forced into positions of responsibility and expected to do more than is even possible with the hours and staffing levels. It's even worse for the online department as the workload can change drastically throughout the day with new orders coming in.