they just ask you to send in copies....
QUOTE
Submit copies of documents indicating that the marriage upon which you were granted conditional status was entered in ''good faith'' and was not for the purpose of circumventing immigration laws. Submit copies of as many documents as you wish to establish this fact and to demonstrate the circumstances of the relationship from the date of the marriage to the present date, and to demonstrate any circumstances surrounding the end of the relationship, if it has ended. The documents should cover, but not limited to,the following examples:
1. Birth certificate(s) of child(ren) born to the marriage.
2. Lease or mortgage contracts showing joint occupancy and/or ownership of your communal residence.
3. Financial records showing joint ownership of assets and joint responsibility for liabilities, such as joint savings and checking accounts, joint federal and state tax returns,insurance policies that show the other spouse as the beneficiary, joint utility bills, joint installments or other loans.
4. Other documents you consider relevant to establish that your marriage was not entered into in order to evade the U.S. immigration laws.
5. Affidavits sworn to or affirmed by at least two people who have known both of you since your conditional residence was granted and have personal knowledge of your marriage and relationship. (Such persons may be required to testify before an immigration officer as to the information contained in the affidavit.) The original affidavit must be submitted and also contain the following information regarding the person making the affidavit:his or her full name and address; date and place of birth;relationship to you or your spouse, if any; and full information and complete details explaining how the person acquired his or her knowledge. Affidavits must be supported by other types of evidence listed above.
http://www.uscis.gov/files/form/I-751instr.pdf