They must be very special golf balls if you're prepared to pay over twice as much for shipping as you do for the balls ;-)
The good news is that you'll have nothing to pay as long as you only import a small enough quantity to stay below the 'cut-offs'.
The way it works is that HMRC start by calculating the import duty. This (for golf balls) is 2.7% of the value of the goods. So, theoretically, they should charge you about 23p. However, if the duty comes to less than �7, it's waived.
HMRC then work out the VAT. This is 17.5% of the sum of the value of the goods + the price of the carriage + the duty. That comes to around a fiver. However, VAT is waived if the value of the goods is less than �18.
Finally, HMRC add on an �8 charge for examining your package. But this is waived when there is no duty or VAT to pay.
Putting all that lot together means that you can normally import goods, from non-EU countries, without paying any charges, as long as the value of the goods is less than �18. (The only exceptions relate to goods subject to excise duty, such as tobacco and alcohol). So you can order one or two boxes of golf balls, at the same time, without any charges. However, if you place three orders simultaneously (i.e. $51), you'll go over the limits and have to pay charges.
Chris