Shipping, duties and customs
The complaint is never the duty. It is the surprise.
Say it before they order
For a cross-border order: whether duty is prepaid or collected on delivery, who collects it, roughly what it will be on that value, and that it is set by the destination country rather than by the shop.
Delivery windows
Give working days from dispatch, not from order, and say what dispatch depends on. Name the cut-off time and the days it does not run.
Restrictions
Check the destination before promising anything for batteries, liquids, aerosols and anything perishable. A restricted item found at the border is a lost parcel, not a delayed one.
When a parcel is held
Say what is being asked for, who has to provide it, and by when — held parcels are returned or destroyed on a timer, and customers rarely know that.
Never
Estimate a duty as zero to close a sale, or mis-describe contents or value on a declaration for any reason.