Dear friend. I am Vtiger Developer. You can check my review or badge to see my skill about vtiger and amount of project I did. I had many experience about this. So I am confident to deliver you best result about this feature you want.
About your project. Have some problems I need to share
1. Why current Invoice : import/export is disabled ? This because Invoice has multi-products when save, you can select multi-product, server , quantity, total price so it is hard to organize information in CSV with this multi- product structure. That is reason Vtiger Developer team disabled import/export/ That iss complex issue
2. You want to export : Select date range, select field from invoice to export so these fields contain field from products in Item Details or only information from Basic Informaton tab.
3/ Support to export within 15 or 30 minutes no need to select again?
4. Support to export and attach via Email?
I am sure about skill and understood what you say. I will do and support until you are satisfied.
Thank you so much and see you soon
Best regards
Linh