Hello @artosdesign
Thank you for reaching out to us; we are here to help.
When you attempt the synchronization, the plugin will show you 2 options for the sync:
1) Add WooCommerce products – Synced WooCommerce products and stock quantities will be added to the existing PayPal Zettle library.
2) Replace PayPal Zettle library – Replace your existing PayPal Zettle library with synced products and stock quantities from WooCommerce.
So how the sync occurs is up to you; you can replace the Zettle library (2) or you can add the WC to the Zettle library.
Your second question is not a question related to the plugin, but the performance of the Zettle POS. You will need to contact Zettle to get an answer to that. Here you find options on how you can get in contact with them: https://www.zettle.com/gb/help/contact/options
Hope these answers help you further. Let us know if you have any further questions.
Kind regards,
Joost