‘And/or’ is an awkward little phrase that most commonly pops up in legal and technical writing. In trying to cover several bases, it actually tends to make the text more imprecise and adds the risk of ambiguity.
In most cases, your sentence will be fine with ‘or’ alone, because it implies ‘or both’.
You must pay our costs if you or your contractors breach this contract.
You must pay our costs if you and/or your contractors breach this contract.
However, sometimes ‘and’ will be the better choice.
You agree to our staff handling and transporting your materials.
You agree to our staff handling and/or transporting your materials.
If you think both options are essential, go with ‘or’ and add ‘or both’ afterwards.
The agreement must be signed by the CFO or the CEO, or both if the project value exceeds $10 million.
The agreement must be signed by the CFO and/or the CEO depending on the project value.
Overall, the ASG suggests you will rarely need to use ‘and/or’. In legal drafting in particular, you are better off specifying more precisely whether you mean ‘and’, ‘or’, or ‘or both’, even if that makes a sentence a little longer.