Walmart offers the Ideal Medium Thermoplastic Pet Door in White, model no. PPDM, for $18.51. (Amazon charges the same with free shipping for Prime members.) Opt for in-store pickup to avoid the $4.97 shipping charge. That's the lowest price we could find by $6. This tamper-proof doggie door is designed for pets up to 35 lbs. and keeps air from flowing out.