Currently, when Apple sends an invoice to its customers and business partners, the recipient gets an invoice number that is entirely numeric and spans 10 digits.
According to a post on Reddit citing a letter from Apple called "Change in Invoice number format," the company has drafted so many invoices that it's close to going through all possible permutations for these 10-digit numeric codes, necessitating a switch to an alphanumeric system.
This means that instead of a basic 10-digit number, Apple's system will now include two alphabet characters followed by eight numbers, for example: "AB12345678." These changes will begin applying to all invoices issued "approximately after the third week of February 2019."
As explained in the Reddit thread on the topic, there are a total of 10 billion possible invoice numbers available to Apple through its current 10-digit all-numeric system, suggesting Apple is nearing 10 billion invoices sent worldwide for as long as it's used this system. Apple sends an invoice every time an order is made on the Apple Store app, website, or in-store, with each order receiving a unique invoice code.
When adding in two letters to the beginning, Apple's invoice code possibilities increase to around 67.6 billion. Redditor nalexander50 explained the math in greater detail:
Wow, that's a lot of orders! If I am remembering my college combinatorics course properly, a 10 digit numeric only code would cover 10 Billion orders. Each position in the number has 10 possible values (0-9) and there are 10 values total. Multiplying it out, 1010 = 10 Billion.
Changing the first two positions to letters vastly increases the possibilities. For the first two positions, there are 26 possible choices (a-z), and the remaining 8 positions have the same 10 (0-9) choices. Multiplying it out, 262 * 108 = 67.6 Billion invoices. Since it has taken this long to creep up on 10 Billion, I figure that an additional 67 Billion will last a while.
With yesterday's update to the Apple Store iOS app, the company also made it possible to get to your order invoices from within the app. To do so follow these instructions: open the app, tap the circular profile symbol at the top right of the Discover tab, scroll down to My Orders, tap on one, tap Print Invoice, then sign in to your Apple ID. On the invoice, the invoice number is located in the Additional Information section at the very bottom of the page.
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EDIT: I misread the original statement as 10 digits + two letters. In fact it says two letters + 8 digits.