I've been selling some of my old textbooks on Amazon lately. Four of the six I've sent off have arrived (the earliest got there the 11th, the last today) but none of the buyers have left me feedback. For the one that got there today, I'm not too worried, but for the book that arrived on the 11th I am. I was thinking of sending the buyer a message, just asking if the package had arrived alright and maybe asking them to submit some feedback. I'm a new seller and would really like to start building my stats. How long should I wait before contacting the buyers?