Fix suggestions

1) calculations
calls like:
get_price()
change to:
get_price('edit')
called with no parameter the price is returned in View mode after filters are applied, these filters may include currency conversion, if you have a converter plugin installed.
If you then do the calculations based on the already converted price you can get some odd things happening like doubly-converted numbers, basically wrong price.
If you call the function with edit then no filters are applied, you get the raw price as per admin edit screens.

2) array testing
change:
$woosb_item_qty = ( $woosb_item_arr[1] ? $woosb_item_arr[1] : 1;
to:
$woosb_item_qty = ( sizeof($woosb_item_arr) > 1) ? $woosb_item_arr[1] : 1;
better to test the size, to avoid error if there is no value…

3)
session_start()
change to
if(session_status()==PHP_SESSION_NONE) session_start();
to avoid session already started notices