Paying in installments is a common practice when buying big-ticket items. However, there are several synonyms for this phrase that can be used interchangeably. One such synonym is "by installment," which means payment is done in regular intervals. Another phrase that can be used instead of "in installments" is "in portions." This phrase indicates that payment is done in smaller portions rather than in one lump sum. Additionally, "in stages" is another synonym that can be used to describe payment done gradually over time. Finally, "piece by piece" or "bit by bit" can also be used to indicate payment done in stages.