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Amazon [amazon.com] has Wilson Championship Extra Duty Tennis Balls (3-Count) on sale for $2.87. Shipping is free w/ Prime or on orders $35+.

From Merchant’s Product Page:[LIST][*]Specs:Count: 3 balls (single can)
Duty rating: Extra Duty
Ball diameter: 6.54 cm
Material: Rubber with woven felt construction
Color: Yellow
Model: WRT100101
[*]Features: