How does Pocket reconcile Shopify/Amazon payouts with recorded sales?

When you make a sale in Shopify/Amazon, that sale is recorded to the account: Sales of Product Income - Shopify (or Amazon). Since you've made a sale, that account has increased. 

The sale is also recorded to another account at the same time: Shopify (or Amazon) Receivables. This is because you've made a sale, and now Shopify (or Amazon) owes you money - they still have to pay out that sale to your checking account. So this Shopify (or Amazon) Receivables account also increases.

Another way to think about this is that it is similar to making a sale to a customer that you've sent an invoice to - you've made the sale, and now your customer owes you money. That customer may pay you in 30 days (or more!). This sale is recorded to the account, Accounts Receivable. 

The nice thing about Shopify and Amazon is that those platforms typically make a payout to your checking account every few weeks (or maybe even days). So you're not waiting 30 days like you might with a customer to whom you've sent an invoice. 

When Shopify and Amazon make a payout to your checking account, this payout will then reduce the Shopify (or Amazon) Receivables balance, because now they are paying part of the balance that they owe you. 

To summarize what happens to the Shopify (or Amazon) Receivables account: it goes up when you make a sale, and goes down when that platform pays out to you. 

The Shopify Receivables and Amazon Receivables accounts are on your Balance Sheet. This means that when you look at those accounts on your Balance Sheet, it tells you how much Shopify and Amazon owe you as of the end of the respective month. Since the platforms don't make payouts immediately, there will be a balance that represents a few weeks (or days) of sales that the platform has not yet paid out to you.