By default, the picker sets the transaction type to “All” so you can search across all of your rental and sale products and services.
If you generally add one product type to an opportunity, you can edit your profile to set a default transaction type.
To edit your profile, click your picture at the top-right and choose Edit your profile from the menu.
On the next screen, choose your transaction type from the “Default Picker Transaction Type” dropdown:
Hit Update User Account when you’re done!
This is a per-account setting, so you can set different transaction types for each user with an account on your system.