Is there a solution to this matter?
When creating custom fields, they must be placed on a (sales invoice) in order for them to appear in Sales Invoices
Your post is not clear. Your title makes an obvious point. Beyond that, you need to illustrate an actual problem. Show a screen shot of the Edit screen for a custom field that does not appear where you want it to.
Before doing that, make sure you have defined a custom field for sales invoice line items, not for sales invoices themselves. And make sure the setup has the box checked to show on printed documents.
From here u can find Line Custom field on Sales Invoice -Lines Level.
One thing which is still missing is selecting Non-line custom fields Sales invoice -Lines (Including other Transactions at Line level too).
Not clear if you want to associate the Passenger custom field with the Sale Invoice or Sale Invoice line?
If you want to include it on the line, then it doesn’t make sense to include it on the Sale invoice as well - after all, you could have two sale invoice lines with different information - so which would you show on the invoice?
Perhaps if you describe what you are trying to achieve, you might get better help
Under Placement you can select as many as available in the list. Note that when adding Sales Invoice -Line that it will only appear on the line items of the invoice and when adding Sales Invoice it will show on the main view. You hve to enter both fields separately when editing the Sales Invoice, one will appear in the line-items and the other on the bottom of the edit screen.