WWII information or direction

I am trying like many others to obtain information on my Dad’s service during WWII. I am particularly interested in units he served with. I have his discharge, which is very limited in scope. I sent in a request this week for any and all records from the National Archives. His discharge may or may not be of help because it is a photo copy and the last digit of his service number is questionable because it appears that it was corrected by the officer who processed his separation.  I also have a copy of his draft card. Anyway here is what I know. He was with the 493rd HAM Co at the time of discharge. He entered service in Oct. 1942 in Dallas Texas not sure where he did basic training but I know he was at Camp Barkley and arrived overseas Jan 29 1945. He was a Tech 4 at the time of discharge.Although not listed on his discharge I have found him listed with the 12th Armored Division 493rd Armored Field Artillery Bat. Medic Detach. He was a mechanic wrecker driver. He served in Rhineland. He was discharged at Camp Chaffee, Arkansas April 1945.  I have two questions. The first is once the national archives has completed their search will they send me an invoice? Second where should i further search on my own using the information that I have? Oh I almost forgot I have a photo of him with his wrecker on the front plate driver side id 493rd Ord on the passenger side is 157th Ord. I assume the passenger side was his co-driver/mechanic. Thanks in advance for any and all help and suggestions.