oh honey, no. Start with Standard Shopping. Here's why, and I promise I'll keep the jargon to a minimum.
PMax is a black box that eats money until it's had its fill of conversion data. For an one-product store, you don't have that. so Google will happily blow your budget on YouTube pre-rolls for people who clicked a "watch ad" button by accident and Display Network placements that look great on a dashboard but sold exactly zero units. You don't have the catalogue depth to absorb that waste while the algorithm learns. It's a vacuum cleaner with no bag - just noise.
Standard Shopping gives you actual control. You can see which search queries triggered your product, you can add negatives yourself, and you can kill the garbage queries before they cost you a tenner. For an impulse-buy product, the search intent is probably narrow anyway - a few core phrases will do the heavy lifting. Shopping lets you find those without handing the keys to a drunk intern named Performance Max.
Once you've got, say, thirty to fifty conversions in the bank and you know which search terms actually convert, then test PMax. By then the algorithm has real signal. some one-product stores see PMax crush it with data. Others find Standard Shopping still wins because the intent is so specific. You won't know until you test, but test from a position of data, not blind optimism.
also - since you mentioned Merchant Centre: are your product titles actually using the exact words someone would type to find this product? For a single-product store, title optimisation is half the game. Don't get clever with your brand name. Match what people search. It's that simple.
