This is my take - others may have another point of view. Creating Perform Logic (PL) is not necessarily straightforward depending on what you're trying to do. It was in the duplicate A/P Invoice checking case but for this I would vote against it because it's more involved and requires a fair amount of debugging.
To answer your question you could probably develop PL to compare BatchNo on the After Read event then figure out how skip to the next line in source file if it doesn't match (assuming header is NOT separate from line). But it's not straightforward. There are all kinds of squirrelly things it does with batches and you end up needing several pieces of PL for several events. I know this from this 1 import I did involving importing into Shipping Data Entry batches. The AP Invoice Dupe checking was a single event with no squirrels. Sorry to put a damper on your hope for a quick and dirty solution.
Here are some ideas:
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0. Create a UDF on AP Invoice Header as a BatchNo shadow field.
What is currently called the BatchNo in the source table you can reference that column for the UDF on the Data tab of your import. Then filter your import selection on the UDF via a Run-Assign operation or maybe the Select tab with a with a Run Time Prompt. Basically when they run the import they would enter the BatchNo where see the UDF prompt at the . For the BatchNo field in Data tab just Replace it - no Run-Time stuff. Then test it all see if this gets you anywhere. The point here is you are using something more ""normal"" for filtering selection instead of the squirrelly BatchNo. If it almost works, what about the thought of turning off Batch Entry altogether and use the UDF alone.
1. Use the BOI to filter your table. Your BOI script can use an InputBox to ask for the BatchNo then connect up to the SQL, read through this table they rarely purge, and export only records belonging to the BatchNo to either another table or a flat file. Then last step is to launch the V/I job in AutoRun mode.
2. Use the BOI instead of V/I entirely:
Same as above but last step bypasses V/I. You write A/P invoices directly through the business object.
3. Use SQL tools to create a whittle down version of another table or flat file:
I once created a VB Script that ran off MAS My Tasks menu. It asked for a UserID through an InputBox, then connected to SQL. It used the UserID to pull specific picked sales orders from a SQL WMS system and then exported that to a CSV file (but it could've been another table). Last step was launch the V/I job in AutoRun mode.
If this is not your cup of tea, see if client's IT will take it up since it's all SQL stuff.
4. Use Access to to create a whittled down version of the table to a flat file.
A number of people prefer Access as their tool of choice.
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One more thing. In your particular case, you have the option of getting your pvx development team to quote the PL needed and compare it with price of the external process.