Hi PF_1 , apologies for not jumping in here, this week has been very busy for me so far. I will make a brief comment.
MYOBs pricing options do not cater for your particular business rules, as you have found. Changing the customer's card file for large orders is the most efficient process, but as you say, you must remember to change it back.
You have 4 choices:
- Apply the rules manually
- Find an add-on program which has better pricing options, eg Web Ninja (one of many B2B e-commerce programs). See Apps Search | MYOB Apps.
- Find a different core program
- Change your pricing rules.