Here's what you do:
1. buy the scissors.
2. Borrow scissors from your future self to open scissor package.
3. Thank your future self and wave him/her goodbye.
4. Use your time machine to travel back and let your past self borrow your scissors to open scissors package.
5. Go back in your time machine and wave to your past self goodbye.
If you did this right, you should have your package open.
1. buy the scissors.
2. Borrow scissors from your future self to open scissor package.
3. Thank your future self and wave him/her goodbye.
4. Use your time machine to travel back and let your past self borrow your scissors to open scissors package.
5. Go back in your time machine and wave to your past self goodbye.
If you did this right, you should have your package open.