Why the conversation goes badly when unplanned
An increase arriving without warning feels like an imposition, even when it is entirely reasonable. Costs do rise, and a provider who never adjusts will eventually reduce something else instead, usually attendance time. The problem is the surprise rather than the increase, and it is caused by nobody having agreed a mechanism at the point when both parties were willing to discuss it calmly and neither party ever raised the subject while the relationship was new.
The other failure is silence. Some providers avoid the conversation for years and then request a large correction, which is far harder to absorb than a modest annual step. Ask each bidder how they have handled this with existing clients. A provider with a regular, predictable approach is easier to budget for than one who leaves it until the position becomes untenable and whether the approach they describe matches what you can budget for.
