Business in France buying items on behalf of clients?

I am a wedding planner and have been in business in France for three years and have just posted this question on a wedding planning forum looking for help, but it would seem that it remains a grey area for some. Here’s a copy of my post:

1. Up until now I have only ever received money from clients for my fees (they play all suppliers direct) and have refused to accept money for buying items related to the decor; I source the items (ribbons, table runner, candles, table numbers, fans, pom poms etc.) on the Internet and get the client to pay with their card and have them delivered to me. However, I have a client that wants me to handle the sourcing and buying of all the little bits and pieces for their wedding next year, as they live in the US and are incredibly busy. As I work under the auto-entrepreneur business system I have been led to believe that any money that comes into my business account is considered as income, which means I will have to pay out 25% in charges. This client is happy to give me a float (paid direct into my business account) and for me to give them the change (cash or bank transfer) with the receipts when I see them in July. I am a bit reluctant to do this though, as I am not quite sure how the system works? If anyone has experience of this (the client setting up a separate bank account in their country and giving me a card for example?), and can offer some words of wisdom, it would be greatly appreciated.
2. Do you think it would be possible for me to accept large amounts of money on behalf of my clients to pay suppliers (tent and hire companies) with. This could mean handling up to 80,000 euros in some case? It would be useful if I could as a lot of my clients are not based in France and have to pay high transfer rates, every time they pay a supplier. I also have to sometimes write security cheques on their behalf as well (uncashed and returned after the wedding, when the client has paid for breakages of plates, glasses etc) as they don’t want to raise banker’s cheques on their own country.