Alex, I think Shopify is an excelent platform. Think about how many orders are created on Black Friday with Shopify. Talking about community, I'm from Romania, it's 6AM in the morning and I'm writing to you.
I think 12.9 for an invoicing app it's nothing for a store with multiple POS devices, really we are talking about 3x coffes for 1+y of development effort.
Let's not exagerate.
Have you used other CMS'? Shopify is by far the worst, least customizable, most restrictive, most payed 'additional' features, most useless customer service and by far the most money hungry platform I've ever had to unforunately use.
If you simply read this thread you decided to post your payed app in, you would see that the feature your app is build to add, is one that was REMOVED by Shopify for no apparent reason, against the clear will of their customers.
While your price isn't the issue here, its the fact that the only solution left to re-aquire the functionality of simple order prefixes for sales channels is now a 3rd party payed one.
Where do you get off thinking people would be happy to pay for you app in a thread where they're complaining about losing features of their system they pay monthly for?
To clarify, I have no issue with you or your app, it has a purpose and the price point isn't an issue. The root cause of the existance of your app is what I'm frustrated with.