Address2/Suburb field; using it for Shipping but not for Billing

I've been poking at this for several days now and have not found any similar posts. I use the Address2/Suburb field to collect a 'Store Pickup' location (based on an add-on module I downloaded from someone a while ago). It all works great except the customer is able to select different Store-Pickup locations because there are different screens for Billing/Payment and Delivery/Shipping. This is confusing because tax is calculated based on the Store Pickup location that is selected on the Delivery/Shipping screen. I have been trying to find a way to either default the Billing/Payment address format to not include this 'Address2/Suburb' field, or to have it automatically default to the information from the Delivery/Shipping address. But I can't seem to affect it (either with display of the Billing address information) or with the format (address format appears to be set at the country level and is used by all address functionality within zencart.

Anyone have any suggestions on what I might try? I even started down the path of someone's addon that combines the billing/shipping into one screen, but I haven't been able to get that to run right (hangs on the checkout screen).

Interested in anyone's thoughts on this. Thanks in advance -

It might be simpler to use this alternative store_pickup module which has built-in support for allowing the customer to choose the location they wish ... without having to muss with Address Line 2 things as you're doing now.
http://www.zen-cart.com/forum/showpost.php?p=932860&postcount=4

I need to charge sales tax depending on which store location is selected for pick-up (so this field is tied to zones to know what the tax rate is). The 'Store Pickup' module that is part of the base zencart I don't believe is that sophisticated enough to handle more than 1 store location and tax rates. Do you think otherwise?

The version I pointed you to allows the customer to choose from multiple store pickup locations. However, it doesn't handle taxes based on the selected location.

realized I never logged how I solved this issue - by allowing only 1 address per customer (a setting in configuration), it will enforce the need to have the values the same on both addresses. Perhaps that information will help someone else out along the way.