There are two parts you need to consider in making this decision: 1)the total value of the discount and branding & marketing services to your company and 2) the value of your company.
Part 1) The value of the services is an "opportunity cost" of using this company. If you were to hire another firm with the all of the same qualities, skills and timeline, how much more would you have to spend in cash for the same development, branding and marketing services. Call this figure X
Part 2) Develop your valuation for your company that would use for an investment at this stage. Call this figure Y.
If X/(X+Y) is greater than 12%, then you should do the deal, otherwise you should not.
Example: If all of the services are worth 20K to your company, and you value your company at 140K, then you should be willing to give up 12.5% and would take the deal. If all of the services are worth 20K to your company, and you value your company at 150K+, then you should be willing to give up 11.7% and would decline the deal