It doesn't have to be expensive. Sites that take money on the spot do better than ones that ask customers to call a phone number or print an order form. They will move onto another website searching for a better deal. Your web address will fall into the black hole. Something will probably happen to make them retract their purchase desicision.
Payment seals the deal.
A subtle counter is acceptable but its even better to have your web space provider furnish a detailed monthly report. Track leads and sales obtained from your site so you can monitor succes and make adjustments. Keep the colors and design the same, or similar throughout the site. Use the same icons throughout and place them in the same location on every page. Keep an eagle eye on the competition to make sure you haven't fallen behind.