What to ask, and why

Other vendors sprayed and left. What should be different?
Good work starts with finding the source: drains, past leaks, previous-tenant residue, or shared air handling. Treatment should come after.
Can the work happen while the business is closed?
Usually. Treatments requiring vacancy are typically scheduled after hours; ask when you get quotes.
The smell seems to drift in from the suite next door. Can that be proven?
Tracing the pathway (a shared plenum, wall gap, or drain line) should be part of the investigation. Fixing a neighbor's source is the landlord's call.
Will employees smell chemicals the next day?
They should not. Ask how ventilation and re-entry will be planned so the space smells neutral when staff return.
Why won't anyone give a price over the phone?
Square footage, the source, and the treatment method all change the cost. Expect a walkthrough first, then a written number.