Whether the fee is legal or not I can not answer, you'd have to ask a local attorney. I did check some sources which said that Delaware really does not have restrictions in place as many states do on this issue, but I can not confirm that since I'm not licensed in your state.
I will however answer the discrimination portion of your question (since that's federal and uniform in all 50 states). The only illegal discrimination is that which is based on race, religion, ethnicity, age, gender, sexual orientation, disability, and familial status. Discrimination on any other basis is not unlawful i.e., income, looks, anything other than these protected categories. Some states have additional protected categories, but you'd have to check with the local attorney.
We do not have an attorney-client relationship. I am not your lawyer. The statements I have made do not constitute legal advice. Any statements I have made are based upon the very limited facts you have presented, and under the premise that you will consult with a local attorney. This is not an attempt to solicit business. This disclaimer is in addition to any disclaimers that this website has made. I am only licensed in California.
I agree with Ms. Sargeant. Check with a local attorney, or a legal aid clinic, to make sure of Delaware's laws regarding drug testing as a prerequisite to employment.
This response is meant for general information only and does not constitute legal advice. This response does not create an attorney-client relationship.