How to buy straps....
Make a list of what you think you might need
Think about it, remember 4 you forgot, add to list
Think about it some more, adjust list, order
Get the order and start prep rigging in garage
Figure out you need 4 more of this, six more of that, and 2 more really long ones
Finally go on a trip
During the trip discover that 2 of those and 4 of those would have been really handy
Get home and decide you want to tweak how you rigged from last trip to new trip
New rigging plan will require 3 more of those and 2 more of that
Repeat until every time you decide you change, remove or add something you have twice the number you need.
That's more like it went for me anyway...
and fwiw since I haven't seen them mentioned, a king sling, like the everything bag, is another great and convenient way to secure the stern gear pile. really like mine.