是30天的平均值。但是取样是在23-27号之间，并不是invoice结束的时候。
Check below to see if you are meeting discount requirements
eBay uses the criteria below to determine the discount on your next invoice.
eBay determines your discount level between the 23rd and 27th of each month
before sending your invoice. Click the drop-down menu to view requirements
for each available discount tier.
