错误在于句子中的"or much"应该是指"or more"，表示12个托盘或者更多的情况下。另外，"L/C"应该是"L/C"，即信用证（Letter of Credit）的缩写，前面不需要加"irrevocable"这个词，因为信用证本身就具有不可撤销的特性。
正确的句子应该是：
On receiving orders for 12 pallets or more, we will give a special discount of 3% for payment by L/C reaching us 20 days before shipment.
正确的句子应该是：
On receiving orders for 12 pallets or more, we will give a special discount of 3% for payment by L/C reaching us 20 days before shipment.
