pcs and unit

When you talk about shipment volume of a certain product, which one of the followings is more appropriate?

- 30,000 pcs per year
- 30,000 units (or unit?) per year

Thank you.

30,000 units per year

So should I avoid using "pcs"? Or is it still acceptable?

No.

30,000 PCs or 30,000 units

The above "PCs" looks like a plural form of "PC" (personal computers). True?When I wrote "pcs" above, I used it as a shortend form of "pieces". Is this abbreviation not used commonly?

You can use it as long as you put a period at the end. Abbreviations should have periods to show that they're abbreviations.