Nick here from Shopify.
The best way to set up a payment plan or monthly options would be via an app from the Shopify App Store. It's not a feature which is available natively within Shopify.
Whether or not these would be able to work with the POS is something that is best to check with the developers of the apps if this is possible. You can see some of the most popular and used payment plan apps below:
The app developers would be the best people to reach out to as they developed the apps and would know the full capabilities and features of them all.
Another possible workaround, if this isn't possible which isn't completely ideal, would be to create draft orders in your online store in person with your customers.
All the best, Nick
Whatever you do, avoid Partial.ly - some payments won't register with Shopify and their response, days later, will be: We don't know why that happened but you can manually enter it into Shopify. If you love headaches and poor customer support, then by all means.