At the top of the union issues forum page is a thread with the National Master Contract between the Transfers and UPS. Download it and look it over. Here is the language pertaining to your question.
Section 2. Union Shop and Dues
(a) All present employees who are members of the Local Union on the effective date of this Subsection or on the date of execution of this Agreement, whichever is the later, shall remain members of the Local Union in good standing as a condition of employment. In order to assist the Local Unions in maintaining current and accu- rate membership records, the Employer will furnish the appropri- ate Local Union a list of new employees. The Employer agrees to notify the Local Union when a new employee attains seniority. This notifcation will be made in conjunction with the new em- ployee listing. The list will include the name, address, social secu- rity number, date of hire, hub or center to which assigned, shift, and classifcation or position hired into. The Employer shall also notify the Local Union when the employee is promoted from part- time to full-time. The list will be provided on a monthly basis. All present employees who are not members of the Local Union and all employees who are hired hereafter, shall become and remain members in good standing of the Local Union as a condition of employment on and after the thirty-frst (31st) day following the beginning of their employment, or on and after the thirty-frst (31st) day following the effective date of this subsection, or the date of this Agreement, whichever is the later. An employee who has failed to acquire, or thereafter maintain, membership in the Union, as herein provided, shall be terminated seventy-two (72) hours after the Employer has received written notice from an au- thorized representative of the Local Union, certifying that mem- bership has been, and is continuing to be offered to such employ- ees on the same basis as all other members, and further that the employee has had notice and opportunity to make all dues or initi- ation fee payments. This provision shall be made and become ef- fective as of such time as it may be made and become effective under the provision of the National Labor Relations Act, but not retroactively."