If you have placed your order in an 'on hold' fulfilment state within Shopify, then this order will be not be available to view within either the 'Incoming orders' screen or the 'Created labels' screen of the application.
Once the fulfilment has been released back to an 'unfulfilled' status using the 'Release fulfillment' function in Shopify, this order will reappear back in the 'Incoming orders' screen ready for your shipping labels to be purchased.
I have already purchased a shipping label before 'holding' an order
For orders where a label had previously been purchased before placing that order 'on hold', this label will have been retained by the application and can be accessed from the 'Created labels' screen once you have 'fulfilled' your order in Shopify.
For example, order #1021 has had its fulfilment released in Shopify:
This will first appear back on the 'Incoming orders' screen as an order which is being processed:
Once this order is marked as fulfilled again in Shopify:
This will again be available in the 'Created labels' screen where you will be able to 'Print label':
Best Practise Guidance
Based on the current functionality within Shopify and the application - cancelling a fulfilment for a previously "Fulfilled" order and placing it "On hold" will mean that the tracking link for the order will be cleared and will no longer be available in the application. For this reason, best practice advises orders are placed 'on hold' prior to labels being purchased.
Any tracking link previously received as a part of a fulfillment email distributed to your customer will still however be operational.