You can also reserve tickets by phone (usually for 3 days) and send payment by cheque, postal order or theatre tokens, enclosing a stamped addressed envelope, or pay in person at the theatre. Tickets will not normally be held until the day of performance unless paid for.
Some theatres and agents now claim to offer online ticketing. As yet this is not actually online, but technically an email postal booking. You will be asked to fill in a form with your requirements and credit card details, and email it to the theatre or agent. They will fulfil your request and email you confirmation. Tickets will be posted out if there is time or held for collection.