Direct REST countTokens billing for Gemini 2.5 Flash multimodal input

Could a Google representative confirm the billing of this direct Developer API
endpoint (not Vertex AI or Firebase SDK)?

POST https://generativelanguage.googleapis.com/v1beta/models/gemini-2.5-flash:countTokens

We use generateContentRequest containing the exact Standard generation request
with inline video and audio. Firebase documentation states Count Tokens is free.
Does that also apply to direct Developer API REST requests for gemini-2.5-flash?
If not, what rate and billing unit apply, including unsuccessful requests?

The intended test is at most two countTokens followed by two generateContent
requests, zero retries, no tools or explicit cache, thinkingBudget=0 and
maxOutputTokens=4096 each. Generation stops if the combined input count exceeds
120000 tokens. We want the complete cost to remain within EUR5.

Which official document establishes the applicable conversion to EUR and
mandatory taxes or surcharges for a Belgian customer? We understand private
account conditions may require a private billing channel; please identify that
channel rather than requesting account information publicly.

Please link an applicable official pricing reference or identify whether your
answer is a community interpretation rather than a provider confirmation.