How to make an invoice

[manoskok]

I need help how to design an invoice for customers applying for chemical analysis

Each customer has specific price for each chemical substance

Each customer can apply for certain chemical substances

I have a table where the analysis is recorded :
customer name, date, code of analysis
substances examined
value for each substance

I have another table where the cost is recorded :
customer name
substance name
substance cost

I don't know how to evaluate the total value for each analysis (ie for the specific customer, to identify which substances were examined,find the relevant cost for each substance, add the costs to find the total for that analysis and find the total cost for all the analyses for that customer)

Please help