I want to submit I-134 for my relative's B2 visa extension;in part2 section of the form should I provide my US address for mailing/physical address?

The choice is up to you. Unlike the I-539 extension form which asks both for US physical address and US mailing address, the I-134 form asks for a US physical address, but does not ask for a US mailing address – just for the mailing address. As the I-134 affidavit of support form is only a part of the I-539 filing, the mailing of notices from USCIS is not dependent upon what is indicated as a mailing address in the I-134. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.