Immediately verbally accepted a job offer without negotiating salary; too late to inquire?

I recently received a job offer and the hiring manager gave me a salary range for the position. Today an HR person called me with an exact number in the upper range of the estimate and asked me if it was OK. I was happy with it and immediately accepted. Afterwards I was curious and looked up my job title on glassdoor and found that my new employer's competitors actually pay about 15% more. By having told her the salary was fine, did I immediately close my window for salary negotiation?

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