If for whichever reason collection through the Payment Service should fail Tourstart will invoice you by post for the amount payable which must be paid by cheque postal order or credit card within 15 days from the date of invoice

Please fix the punctuation.
If, for whichever reason, collection through the Payment Service should fail Tourstart will invoice you by post for the amount payable, which must be paid by cheque, postal order or credit card within 15 days from the date of invoice.