Don't you have an hourly labor charge?
What I have has changed over time, and still trying to arrive at a charge methodology I am comfortable with. Hrly labor charge works fine for longer duration efforts, but not really for short duration efforts that involve a bunch of other busy work. Generally, the process for me works like this. The customer calls and I need to get basic info from them on the issue. Usually they come from an installer that went out of business (i.e. Power Home Solar), or from an installer that stopped responding or mistreated them. I typically email them info to transfer monitoring portal access to my company account, so I can see their system history online, then I either open the case and get inverter ordered, or they have already done that and physically have it. Then schedule the work, typically 1.5h onsite. Drive time varies between 20min and 1hr 1-way, sometimes longer. Then, sometimes SolarEdge will have a small labor reimbursement, which requires filling out reimbursement request, emailing an invoice to them, and tracking it over a few months til it's paid. But you don't know for sure about reimbursement because they tie it to how long since the commission date and whether it is the 1st or second time the inverter was replaced.
I want to be fair for the customer, since their system is down and they are upset, but it is more work than the 1.5h onsite, especially if something goes wrong and I need to call SolarEdge Support, which adds at least one hr minimum as they usually are not available and you wait for a call back. While doing this, I am not working on design or install for new systems, which result in a lot more income. Just curious how others are doing it.