There are two important concerns other than the profitability analysis as suggested:
1) Does the service have a material cost attached?
For the communication tool development, if the tool needs to be customized for each sales of consulting service, a pay-per-sale pricing model is justified. If the tool is a general model and needs minimum tweaking after the initial R&D and implementation, it may be more sensible to have a bulk one-time initial charge or a subscription model in order to minimize the impact of potential sales fluctuations.
Similarly, for the maintenance service, if the cost of service is primarily attached to the sale of communication tool (i.e. maintenance service is only provided as part of the consulting service project), a pay-per-sale pricing model is more suitable. If the maintenance service is spread out such that it is carried out regularly during the year (e.g. once a month), a monthly/annual fee is more justified.
2) Shifting to subscription model in the long run
In the short term, pay-per-sale model is a flexible option that provides feedback on the client's business performance.
If the client consistently has a low sales number, it may not be profitable for the company to continue providing the communication tool service. Potentially, the company may consider ending this engagement, making a few adjustments to the communication tool and selling it to other interested third-party clients.
If the client has a steady sales number that generates reasonable profit for the company, the company can switch to a subscription model in the long run to secure consistent revenue streams and better capture customer lifetime value by maintaining customer relationships.
The pay-per-sale model is a 'testing ground' to probe into whether the engagement is potentially a profitable one. The conversion to subscription model can maximize the future revenue potential based on the testing results.
Hope this gives you another perspective of the case.