The basics:
You are in a frioegn country. Be cool, be respectful.
Dont draw attention - especially while at the Vet or Pharm.
Dont buy a LOT of stuff, just a reletiveky small amount.
Dont park your car within view of the Vet/Pharm. If you walk out and into a car, its easy for someone to make a connection between you/gear/car and report ya to the border.
Change clothes/look after you viist the Vet/Pharm before going to the border.
Dont looked really different "big beard" or whatever - again, you will stand out.
Assume this ...... if you walk into a Vet/Pharm and walk out into a car which is within view, that someone may see you and call the border. Now, they might give a brief description of car, clothes, any traits about you which are different/unique.
So dont be different or unique. Change your shirt (maybe in the car on the way towards the border). Park way from Vet/Pharm.