Starting Amazon FBA with one product can work, but please be careful because a lot of “done for you” agencies oversell the dream
A few things to look out for;
1, Ask if the product is already validated or if they’re just guessing it will sell. Huge difference.
2, $700 PPC for launch is possible on Amazon these days, especially in competitive categories. But if the product only survives with ads and has no organic ranking strategy, profit can disappear fast.
3, 25 to 35% profit sounds good on paper, but ask if that’s AFTER Amazon fees, PPC, shipping, refunds and storage fees. Many people quote margins before expenses.
4, Make sure the Amazon account stays fully under your control, your email, bank, LLC and brand registry if possible.
5, Ask what happens after 3 months. Will the product rank organically enough to reduce PPC, or will you need ads forever? Most products still need some PPC, just usually less over time.
6, Be careful of agencies making money mainly from monthly retainers instead of successful products.
If possible, ask them for real case studies, screenshots from Seller Central and proof of long term profitable products, not just launch results 
Amazon FBA is definitely possible, but go into it like an investment business, not passive income from day 1.
Do you understand better?