Minor UI changes

Added $ sign for cart output
Commented cart output
Changed display message for case of adding item to cart that already exists
Changed display message for default case of command interpretation
Fixes to follow SOW

- Changed type of ArrayList per SOW
+ Command echo added per SOW
- Changed type of items TreeMap to hold Integer value per SOW
Corrected warnings after linting

Warnings caused by not explicitly declaring parameter object type for data structures throughout entire program.  Fixed all.