If you are used to do this, what's the usual, safe way?
I ever trade goods with altcoin as payment. First, you need to know how much the shipping cost, usually the post office have a website which you can estimate your shipping cost.
For the calculation I used fiat or USD rate on the transaction, then convert it to coin price, use seller fees to avoid the volatile price, I set 50% of total payment for seller fees, so the total payment = price product + shipping cost + seller fees (exclude escrow fees).
In my experienced, selling(if you are a merchant) goods in cryptocurrency as payment require extra funds to cover the volatile price.
Not all buyer agree with the seller fees.