I'm going to make a few assumptions based on what you have said, trying to fill in gaps of what you didn't mention. If I have guessed wrong please say...
You used the term "toll tag" which sounds American so I presume you originating in the US or Canada?
It sounds like will be driving alone.
It sounds like you are using the Eurotunnel.
If so, if you take the first exit to the right and then turn left and follow your nose to Cité Europe just past the Ibis/Ibis Budget/Novotel hotel complex you will find a 2 storey car park and a very large - by French standards - shopping centre. The journey from Folkestone to Coquelles is only 35 minutes on that train but you will start boarding 20 minutes prior, and you will be in the Departure lounge before that waiting for your crossing on the big board. Leave plenty of time, especially with the "interesting times" that Brexit is bringing.
Are you sure that your rental company allows their cars into France, and on the Eurotunnel Shuttle? Quite a few don't. You will need special paperwork.
It is possible to rent (not buy) a bipper from the UK representative of the tolling authority but it won't be easy or cheap. I have one but I make a lot of trips with my own car. You will need a deposit, an annual fee, a fee for each calendar month or portion it is used, and probably hardest of all, a UK bank account in which you can set up a Direct Debit.
If you are on your own with the wheel on the wrong side for French driving you will need to fit enough extra mirrors to account for all blind spots and you will find it difficult to pass. Experience speaking here, I've been doing it for 20 years or so but the first few trips were fun.
And you will need help or getting out of the car and walking to the toll machine not only for paying but also for getting the start tickets. You want to be in the cash lane and most rural toll plazas only have 2 lanes. You will be making friends with the people behind you so I advise developing a thick skin.