self.price = printf('$%.2f',totprice)This does not set the price, it prints it. printf is for printing and returns nil so that does self.price = nil. You want sprintf.
If price is a number you probably don't want to add a $ to it. Generally avoid adding formatting to numbers in your database, it makes them harder to work with. In that case, use Numeric#round.
self.price = totprice.round(2)Notes
@aand@bare instance variables on the object which will persist. You probably don't want that, and you risk accidentally overwriting other instance variables such as@price. If you just need a local variable in a method useaandb... tho don't useaandb, use descriptive names. See Read This If You Want to Understand Instance Variables in Ruby.- It's generally not necessary to check
if cert == true.if certis sufficient.
Consider using rubocop and rubocop-rails to guide you.