Advice, please, on which custom fields to use

I have an offer to subscribers like the following:

1 subscription = $200

More than 1 subscription up to 5 subscriptions = -10%

Optional added service = $85

Can I use both Dropdown and Multiselect and add the 2 choices together for the Total Payment amount:

Dropdown would let buyer choose 1 through 5 subscriptions

Multiselect to add optional service.

Must I use 6 multiselect boxes