Hope i got your question right.You created a custom menu admin/commerce/orders/*/printlabel where * is the order id of a particular order. You need the order id.
The simplest solution to it is getting it through the arguments or arg(). This is a function that drupal provides to get data from the arguments. For eg
if your url is www.xyz.com/admin/commerce/orders/0001/printlabel & you need order id i.e 0001 you can get it using arg(3).