In addition to the information of the other members, I would like to explain my vision on how I would handle this.
You gave 4 examples, which is good to have an idea about the products. The thing is, that they are totally different for your sampling plan. So it may be an advice to divide your products in different groups and develop sampling plans per group. Hereunder you find an explanation, which can be more nuanced according to the context:
"Examples: (Lettuce, carrots, blueberries, Canteloupes)"
Lettuce: Comes from close to the ground. It's washed and consumed in a raw form. There are probably ground bacteria, such as Listeria and Enterobacteriaceae (often the type Pantoea agglomerans) on it. Furthermore, you have to note which fertilizers were used (valid for all the examples) (to avoid contamination with the dangerous pathogens like E-Coli 0157 and Salmonella). You can also monitor that hygienic conditions were applied, during the cultivation (= verification of supplier audit and questionnaire)
Carrots: Grow in the ground, so from a microbiological perspective worse than lettuce. What you need to look at is whether they are consumed raw or cooked?
Blueberries: Characteristically high numbers of yeasts. These fruits are not peeled and can be consumed raw. So also important there is to look after an eventual contamination during harvest, particularly Enterobacteriaceae. As above, you need to know what is done at the cultivator, whether good hygienic conditions (irrigation water, hands touching the fruits and cleanliness of the crates) are respected. What you do at your incoming goods control is to monitor this.
Cantaloupes: A fruit which grows nearby the ground. But, in contrast to the previous examples, this fruit is peeled. Although that the peel is not consumed, you have to be sure, that there are no pathogens on the peel, which can contaminate the inside of the fruit.
I hope that the above gives you an idea on how to build up your sampling plan. It's mainly looking to the origin of your raw materials (with its risks in the chain), the process and the intended use of the final product and combining this with microbiological knowledge.