If the invoice shows your product as a service instead of goods,It is possible that you have mentioned the tax category for services in the item's master instead of the tax category for goods,to resolve this you can follow given steps:
Step 1:To open an item in modify mode,press Alt + M.
Step 2:Under the tax category, press Alt + M and open it in modify mode.
Step 3:Here you can see in the type feild you must have mentioned service,there you will need to select goods instead of service,save this window and item master.
Now,your item will be considered as a goods.