if you want to prevent running multiple instance
of the same c# application then check the
following lines.It's sometime necessary to prevent
running multiple instances of the same application.

command objects use parameters for passing values
to sql statements for type cheking,validation,guarding
against SQL injection attacks,etc.so its a good practice
to use command parameters to sql statements.

you can use online paypal buy button in your page
for receiving online paypal payment.when users click
this buy button they will enter in payment page
for payment.