You have mentioned that in your sales office, there are fewer than 10 employees.
Applicability of the Act
The act states that it applies to any establishment with more than 10 employees.
Definition of "Establishment"
Sec 2(e) of the act defines "establishment" as:
(i) a factory;
(ii) a mine;
(iii) a plantation;
(iv) an establishment where persons are employed for the exhibition of equestrian, acrobatic, and other performances;
(iva) a shop or establishment; or
(v) an establishment to which the provisions of this Act have been declared under sub-section (1) of section 2 to be applicable.
Since the sales office probably has a shop and establishment license, it is covered in the definition above. If it has fewer than 10 employees, I suppose it is exempt.
State-Specific Considerations
However, you also need to check the shop & establishment act of your state because some states have included additional clauses extending the applicability of the Maternity Benefit Act to offices with fewer than 10 employees.
Management's Willingness to Comply
Despite the exemption, if your management is willing to comply with the law, that is commendable. You cannot make an assumption, so you should officially inform them of the cases and ask whether they would like to cover the employees even if not strictly applicable.