....especially in light on the current CP issues.
RELAX! The may be some effects but nothing to serious for most goods.
1 - Shutdowns historically have only lasted a few days, they was in the fairly recent past (2018/19) one that lasted for 35 days.
2 - US Customs are deemed an essential service and thus will continue to operate
3 - USPS is also an essential service and will operate normally
Now for the negatives; Some Customs processing is dependant on other Governement Agencies (FDA, Dept of Agriculture etc.), those may be affected as those agencies will be non-function in whole or in part during a shutdown.
Relative to the other issues Canadian sellers are dealing with this seems like a very minor concern. Of course the threat of a shutdown is pretty much posturing at this time, I don't know the current odds of it actually happening.
