adding the "sort by prices" dificulties

I am posting this here, because i was not sure where else to position this.. :/

The questions are regarding the Sort by Price Package modification in functions.php.

1) My site is running Directory starter, with Supreme, and i happen to run in to difficulties with this.
Right after adding into functions.php i receive the following notice:

unable to handle this request.
HTTP ERROR 500

So i had to restore the server app (cpt. obvious…)
Any ideas how to avoid this?

2) Currently i am assigning the corresponding annual and monthly plan to one position, but that might not be wise and cause conflict?
2.1) Is there any more elegant way to offer monthly or annual billing, without creating a separate price package?

Thanks a million again!