The taxes are not coming up at check out when someone buys from me.

[clarksretail]

The taxes are not coming up when my buyers goes through the checkout process. I have added them into my site preferences and included my business number.

[reallynicestamps]

In the USA many sellers use UPS or other couriers for delivery and they cannot deliver to PO boxes. The PO staff will not sign for the parcels.

Other sellers then copied them unthinkingly.

That is why so many US sellers do not accept PO Box addresses.

However, if you are shipping through the postal system, a 'real' PO box is very secure, since the boxholder needs a key to retrieve small items and must go to the counter and sign for larger items. 

This may or may not be true of 'supermailboxes'.  Opinions differ. My colleague has a supermailbox but they use their street address. Canada Post suggests adding the box number for the convenience of the carrier, but do not require it. She tells me she has had transactions refused when she gives a box number as part of her address, so she just changes Box # XX to Unit #XX.