Filtering data on year prior to year selected

First, here is a link to a great tool that formats DAX for you to make it easier to read.

You don't need two CALCULATE statements.  You previously created the measure for [# of Customer Shipments] so plug that in like this.  Generally speaking you NEVER want to repeat the same DAX because if it ever changes, you have to find everywhere it's used to correct it.

CALCULATE(
	[# of Customer Shipments],
	DATEADD(
		'Calendar'[Date],
		-1,
		YEAR
	)
)

Try an experiment.  Take the [# of Customer Shipments] and [# of Customer Shipments PY] and drop them both into a column or line chart, calendar year and month on the X axis with no date slicer.  This will show you that it'll work 😉