I am not sure about town halls in Iowa, but I have found that most in New England are willing to look something up if you have:
Name of person
Name of spouse (bride or groom in a marriage cert)
Approximate year
I generally utilize the same type of form letter to all town halls (I also keep a separate address book specifically for genealogy, which includes not just addresses and telephone numbers, but also costs and other items, such as "requires money order; does not accept personal checks").
Try something like this:
Dear __________:
I am writing to request the following record for genealogical purposes:
The marriage certificate of Joseph O. Johnson, my grandfather, and Katherine Joann Ferguson. Joseph was born in Iowa, in 1928. Katherine was born in _____, in (year). The marriage occurred approximately (year to year).
Enclosed is a check in the amount of (whatever their fee is) and a self-addressed, stamped envelope to facilitate this request. Thank you for your time and assistance.
Sincerely,
The SASE is a must, as I am sure you know.

Then the folks in the Town Clerk's office (or county office or county courthouse - wherever you must write), know they are dealing with a considerate and knowledgeable genealogist.
An approximation won't deter them from checking. I've had many people check approximations for me, sometimes as small as 1925-1935, and sometimes as large as 1800 - 1930!
Good luck. I have my fingers crossed for you.