CanI be arrested for a DUI if I was not driving?

Yes you can be arrested for a DUI even if you are not driving at the time.  The fact is, the police do not have to actually observe you behind the wheel of a moving vehicle.  Under the law all that is needed is evidence of driving. However, in your instance they most probably observed you pulling up to the liquor store. Also, you may have said something about driving to the store when the police stopped you (you didn't indicate). This would have all been legally sufficient to demonstrate that you exerted control over your car. Additionally, I don't know what, if anything, tests were administered to you at the scene (breathalyzer, blood, field sobriety) would further legally establish that you not only exerted control over your car but did so at a time when you were legally intoxicated.

All that having been said, this does not mean that you don't have legal grounds on which to fight the charge.  Defenses that can be enlisted in your case which could result in getting the charge reduced or dismissed.  Right now you need to consult with a DUI attorney in the area where you must make your court appearance.  They will have contacts within the local court that they may be able utilize to your advantage.  Since a DUI triggers both civil and criminal cases, you really should have legal representation.

Note: PA has something called "ARD" which is a special probation. If successfully completed you can be left with a clean record.  However, you only get it once. So you should fight this charge if you can.  This way if you have a similar problem in the future ARD will be available to you then.