A PO is an Order Tracking tool. It allows you to list goods and services and other charge items. It isn't Just inventory.
It can be fulfilled or not. That doesn't matter, because you Decide when you need it or are done with it, such as, they never can fulfill the order, or you issued the PO only for purposes of getting a Quote and won't buy from them.
Let's take your question in sections:
"Often a vendor will ship a different quantity of an item due to shortages or packaging deltas"
When you enter the Purchase transaction for that Vendor, QB points out you have an Open PO for that same name and Asks if you want to show these Relate. They don't have to relate. They can be different activities. You don't need a PO for QB; you would make it for Your own tracking needs.
"and/or the per unit item cost will be different than what was ordered."
You always confirm or update the Purchase transaction for what you got and how many and the Cost. You can also add anything else here, that isn't part of the PO, but is part of this Order you are paying for (write check or enter credit card charge) or a Bill to pay later. Because now, it's just a Purchase transaction for you to complete to match your reality.
"For that reason, I usually wait until I actually receive the items to enter the "receive items with bill" feature"
The "Receive items" provision is to allow you to show they are On Hand for a specific date for Inventory, even if you have no Bill to pay at this time. That's because Inventory that is Sold, has to exist on hand in Quantity available to make that sale, on or before the date of the sale. The Item Receipt is AP, but not yet a Bill = a preliminary Status. It is Changed to a Bill by using the checkmark, top right, or change a Bill to an Item Receipt by removing the checkmark, top right.
If you use "receive with bill" you can simply use Enter Bill. It's the Same thing.
"to enter the actual costs and quantities, even though I paid the vendor via credit card when I placed the order."
Here's what matters: Enter Credit Card Charge for listing Inventory results in the Inventory being On hand as of that date. If you Prepay, that isn't the Items. That is an Other Current Asset account entry on the Expenses tab = Vendor Prepayment. This is Not selecting from the PO, because it is Not paying for the products; it is Prepayment.
Then Later, the products arrive, so this is Enter Bill, and you can select it Applies to the PO and is fulfillment. This is the Gross you owe. Now click on the Expenses tab and put the Other Current Asset here for the Negative, for the amount you Prepaid. Hit Recalc. This is Prepayment Applied, leaving a Net Due, as part of Bills.
It can end at $0, of course, if the entire order was prepaid for the exact amount.
"That works fine other than the transaction dates are not in the usual sequence."
Sequence is Yours to control = use the tools that matter. Match your reality.
"Is there a better or recommended way to use PO's for inventory purposes"
Your Order Tracking tool. If I you call me and we agree you should prepay for my services, you use Enter Credit Card Charge and list Professional Services on the Expenses tab. There is no PO for tracking; you already Paid me and that is what you pay me for. But, if we discuss a possibility of up to 6 hours, and you Prepay me for 2, that is either Prepayment or Professional Services per your accounting requirement, and you could use a PO to track we discussed 6, and you paid 2, already = Order Tracking shows 4 hours yet to be fulfilled and paid.
"but enter a bill associated with the PO (with cost corrections)"
You are not making Cost Corrections. A PO is Non-Posting = Tracking. Cost is from Actual Purchases.
"before actually receiving the items?"
Example: I call you and you quote me $100 for something I will come get. I decide to track this as PO. Then, I come over and give you the credit card. I get back to the office and use Enter Credit Card Charge for what I just paid for. I select that Yes, this is fulfillment of what I put on a PO.
There is no Receive needed. There is no Bill to pay later. There is No AP at all. I already paid you.
"Sometimes an unusual item takes several weeks to receive after its ordered"
You would use PO.
"and paid for."
Other Current Asset.
"I know I could just manually enter a bill from the vendor when I know the costs,"
Only if you Also Have the Inventory as of that date. This is the critical difference between listing Inventory vs Services/Noninventory and Other Charges on the Bill. A Bill is a Posting Purchase. So is Item Receipt; it just doesn't show in the Pay Bills Function, because technically, its status is, "I'm not yet a Bill, so ignore me" but it affects AP. A Check and a credit card charge both as Purchases, as well, so listing inventory here = I have it On Hand as of this date.
"but duplicating a long list of items on a PO without receiving them is too laborious."
You don't need Receive, unless you need Item Receipt as preliminary status to being a Bill. And you never Change the date on the item receipt when you mark that it now is a bill.
You list what you got when you buy it and Have it, for inventory; you can use a PO to track it first, or not. A Prepayment is Never inventory.
Please see my attachment.
