I guess the first problem would be 'getting away' with the travelling expenses, if they specifically want you for the job you will get it, if not someone closer will almost certainly get the job because they will be a lot cheaper. And it is 'only' a tea stain ( which can be difficult, especially if it was hot tea with milk & sugar). So your minimum charge will apply to this job.
Seriously, I don't think there is much profit in this one, but you could very well get the whole carpet, and suggest a protector to help with future incidents, and of course you'll be the person they call for the next carpet clean if you do a good job.
Might be worth walking away from this one
Might be worth doing the job, and waiting for the extra work it could bring.
Everyone is going to charge a different rate, if it was me I'd suggest doing the whole room and charge about £70.00 (presuming its a medium sized lounge). But no doubt, there will be some much bigger price suggestions.
PS. Sorry mis-read, didn't realise job was already completed.
PPS LAst time I ever reply to anything that starts " Hi Dudes"