Okay, I will mention a simpler to implement but sync time expensive way. You could achieve the same with implementing reference actions as well that will not be sync time expensive.
Please add a column called say [Total_Product_Quantity] in the Purchases table with an expression something like
SUM(SELECT(Purchases[Quantity],AND([Purchase_Id]=[_THISROW].[Purrchase_ID], [Product]=[_THISROW].[Product])))
Please note that expressions such as above can take a lot of app sync time as the table size increases. You can of course have some configurations such as using security filters, slices to reduce the sync time but that is a larger subject.
Now please revise the expression for the [Product_Quantity] column as
CONCATENATE([Product], "-", [Total_Product_Quantity])
Please revise the expression for the [Purchase_text] column as
SUBSTITUTE(UNIQUE(LIST("Hi I would like to order: ")+[Related Purchases][Product_Quantity]), ",", "
")
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