Exhibit 10.15

6-1162-ELP-0785

United Air Lines, Inc.

P. O. Box 66100

Chicago, IL 60601-0100

This letter agreement (Letter Agreement) amends and supplements the Purchase Agreement. All terms used but not defined in this Letter Agreement have the same meaning as in the Purchase Agreement.

Boeing and Customer wish to enter into an agreement pursuant to which each party will contribute equally to promotional programs in support of the entry into service of the Aircraft as more specifically provided below.

1.1 Commitment Limit shall have the meaning set forth in Article 2, below.

1.2 Covered Aircraft shall mean those Aircraft identified on Table 1 to the Purchase Agreement as of the date of signing of this Letter Agreement.

1.3 Performance Period shall mean the period beginning one (1) year before and ending one (1) year after the scheduled delivery month of the first Covered Aircraft.

1.4 Promotional Support shall mean marketing and promotion programs in support of the entry into service of the Covered Aircraft such as marketing research, tourism development, corporate identity, direct marketing, videotape or still photography, planning, design and production of collateral materials, management of promotion programs, advertising campaigns or such other marketing and promotional activities as the parties may mutually agree.

1.5 Qualifying Third Party Fees shall mean fees paid by Customer to third party providers for Promotional Support provided to Customer during the Performance Period.

As more particularly set forth in this Letter Agreement, Boeing agrees to provide Promotional Support to Customer during the Performance Period in a value not to exceed [CONFIDENTIAL MATERIAL OMITTED AND FILED

P.A. No. 3427

Promotional Support

BOEING / UNITED PROPRIETARY

United Air Lines, Inc.

6-1162-ELP-0785

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SEPARATELY WITH THE SEC PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT] for the first Covered Aircraft delivered to Customer and not to exceed [CONFIDENTIAL MATERIAL OMITTED AND FILED SEPARATELY WITH THE SEC PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT] per Covered Aircraft for each Covered Aircraft delivered to Customer thereafter for a total of [CONFIDENTIAL MATERIAL OMITTED AND FILED SEPARATELY WITH THE SEC PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT].

3.1 Subject to the Commitment Limit, Boeing will reimburse [CONFIDENTIAL MATERIAL OMITTED AND FILED SEPARATELY WITH THE SEC PURSUANT TO A REQUEST FOR CONFIDENTIAL TREATMENT] of Customer’s payments of Qualifying Third Party Fees provided that Customer provides Boeing copies of paid invoices for such Qualifying Third Party Fees no later than twenty-four months after the delivery of the first Covered Aircraft.

3.2 Notwithstanding the above, at Customer’s request and subject to a mutually agreed project, Boeing will provide certain Promotional Support during the Performance Period directly to Customer. The full value of such Boeing provided Promotional Support will be accounted for as part of the Commitment Limit and will correspondingly reduce the amount of Qualifying Third Party Fees that are subject to reimbursement pursuant to Article 3.1 above.

3.3 In the event Customer does not (i) utilize the full amount of the Commitment Limit within the Performance Period or (ii) submit its paid invoices for Qualifying Third Party Fees within the required time, as set forth in Article 3.1, Boeing shall have no further obligation to Customer for such unused Commitment Limit or to reimburse Customer for such Qualifying Third Party Fees, respectively.

Following the execution of this Letter Agreement, a Boeing Airline Marketing Services representative will meet with Customer’s designated representative to review and approve the extent, selection, scheduling, and funds disbursement process for the Promotional Support to be provided pursuant to this Letter Agreement.

The rights and obligations described in this Letter Agreement are provided to Customer in consideration of Customer becoming the operator of the Aircraft and can only be assigned, in whole or in part, pursuant to Article 9 of the AGTA as amended by Letter Agreement No. 6-1162-IRS-0184.

P.A. No. 3427

Promotional Support

BOEING / UNITED PROPRIETARY

United Air Lines, Inc.

6-1162-ELP-0785

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Customer and Boeing understand that certain commercial and financial information contained in this Letter Agreement are considered by Boeing and Customer as confidential and are subject to the terms and conditions set forth in Letter Agreement No. 6-1162-IRS-0182.

P.A. No. 3427

Promotional Support

BOEING / UNITED PROPRIETARY