You could try A4 self adhesive sticker paper sheets, save using and taping 'normal' paper to the packages (some sellers on ebay appear to have the A4 sheets).

My Dymo LabelWriter spits a label out within seconds, it is thermal label/printing so no need to worry about replacing ink or toner. I simply copy and paste the buyer's address onto the label template I created and hit print! Should have had one years ago. Great for super fast address labels but not sure how it would work for Click + Send etc. I use the 59 x 102mm large shipping labels.